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SOLVENT-FREE OXIDATION OF PRIMARY
CARBON-HYDROGEN BONDS IN TOLUENE USING
SUPPORTED GOLD PALLADIUM ALLOY
NANOPARTICLES
Lokesh Kesavan
C-M <m ICv
Thesis submitted in accordance with the regulation o f Cardiff University for the partial fulfillment o f degree o f doctor o f philosophy
December 2011
UMI Number: U58551B
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DECLARATION
This work has not previously been accepted in substance for any degree and is not concurrently submitted in candidature for any degree.
Signed
STATEMENT 1
(candidate) Date id d .
This thesis is being submitted in partial fulfillment of the requirements for the degree of PhD
Signed (candidate) Date i s
STATEMENT 2
This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references.
Signed (candidate) Date h i) .1 J- U * i 2 -
STATEMENT 3
I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter- library loan, and for the title and summary to be made available to outside organisations.
Signed (candidate) Date b l d .
Acknowledgement
Foremost, I praise and thank god for all the goodness that he blessed in my life. It is his
grace that led me to pursue this PhD degree without any hindrances. He was my provider in
every aspect and he kept me productive throughout the last three years.
I would like to thank my supervisor, Prof. Graham Hutchings FRS, for offering me this
great opportunity and guiding me throughout this project. Further, I wish to say thanks to Dr.
Nikolaos Dimitratose, Dr. Jose Antonia Sanchez Lopez for their immense supervision in this
project.
Further, I would like to extend my gratitude to the academic staffs of Cardiff catalysis
institute, namely Prof. David Knight, Dr. Sturat Taylor, Dr. David Willock, Dr. Jonathan
Bartley, Dr. Albert Carley, Dr. David Morgan, Dr. Rob Jenkins and my colleagues Hasbi,
Mike, Ceri, and Izham for their valuable contribution in my project.
In addition, I thank Prof. Chris Kieley and his students Ramachndra Thiruvalam, Qian
He for their amazing support in terms o f catalyst characterization.
Finally, I acknowledge my wife Liji Sobhana and family members, who were back bone
for me throughout my life and supported me morally.
Abstract
Selective oxidation o f primary carbon-hydrogen bonds with oxygen is of crucial
importance for the sustainable exploitation o f available feedstocks. To date, heterogeneous
catalysts have either shown low activity and/or selectivity or have required activated oxygen
donors. It is reported here that supported gold-palladium (Au-Pd) nanoparticles on carbon or
TiO: are active for the oxidation o f the primary carbon-hydrogen bonds in toluene and related
molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions.
Differences between the catalytic activity o f the Au-Pd nanoparticles on carbon and TiC>2
supports are rationalized in terms of the particle/support wetting behaviour and the
availability o f exposed corner/edge sites. Further these catalysts have been tested for
substituted toluene molecules and benzyl alcohol, as these substrates have similarity in their
structural configuration. The results are discussed with regard to structure activity
relationship.
TABLE OF CONTENTS
CHAPTER — 1 Page No.
Introduction and Literature
1.1. Brief introduction to catalysis 1
1.2. Heterogeneous catalysis 2
1.3. Gold catalysis 4
1.4. Oxidation of toluene 6
References
CHAPTER - 2
Experimental
2.1. Outline 20
2.2. Catalyst preparation 20
2.2.1. Impregnation method 20
2.2.2. Sol immobilisation method 20
2.3. Characterization of catalysts 21
2.3.1. UV spectroscopy 21
2.3.2. X ray photoelectron spectroscopy 22
2.3.3. Transmission electron microscopy 22
2.4. Catalyst testing 23
2.4.1. Toluene oxidation 23
2.4.2. Catalyst reuse studies 23
2.4.3. Benzyl alcohol oxidation 23
2.4.4. A nalysis o f products 24
2.4.5. Toluene oxidation using H2 O2 24
References
26
26
26
28
29
32
33
36
36
38
39
45
51
54
57
Liquid phase oxidation of toluene using Au Pd alloy
nanoparticles supported catalysts under solvent free
conditions
Outline
Reaction optimization
3.2.1. Blank toluene oxidation
3.2.2. Toluene oxidation in the presence o f catalyst supports
3.2.3. Toluene oxidation using mono and bimetallic Au Pd
supported catalysts prepared by impregnation
3.2.4. Toluene oxidation using mono and bimetallic Au Pd
supported catalysts prepared by sol immobilisation
3.2.4.1 Studying the effect o f A u:Pd molar ratio o f
Au-Pd bimetallic sol catalysts upon toluene
oxidation
Attempts to increase the conversion of toluene
3.3.1. Effect o f catalyst mass on toluene oxidation
3.3.2. Effect o f reaction time on toluene oxidation
Toluene oxidation using a lower substrate/metal ratio at longer
reaction times
Structure-activity relationship of Au-Pd bimetallic sol catalyst
system
3.5.1. Calculation of the number of surface exposed atoms
Effect o f temperature and pressure on toluene oxidation
References
Annexure
62
62
69
75
82
83
83
101
107
107
110
114
117
Liquid phase oxidation of substituted toluenes using Au Pd ;
nanoparticles supported catalysts under solvent free
conditions
Outline
Discussion about the formation of benzyl benzoate
Catalyst reusability studies
Solvent free oxidation of substituted toluenes using carbon
supported Au-Pd bimetallic nanoparticles prepared by sol
immobilisation
References
Liquid phase oxidation of toluene using Au Pd bimetallic
alloy nanoparticles supported catalysts
Outline
Solvent free oxidation of benzyl alcohol using supported Au-Pd
bimetallic nanoparticles prepared by sol immobilisation
5.2.1. Results and discussion
References
Conclusions & Future work
Conclusions
Ongoing Work
6.2.1. Effect of amount of solvent medium in sol preparation
6.2.2. Effect of calcinations
6.2.3. Effect of support
6.2.4. Oxidation of toluene using H2O2
References
Chapter IIntroduction Literature
(rlurpter / ^S’ntwduetieu Sc literature
1.1. Brief introduction to catalysis
Catalysis is a process which alters the rate of a chemical reaction due to the involvement
o f a foreign substance called ‘catalyst’. Unlike other reagents that participate in the chemical
reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in
multiple chemical transformations simultaneously. Substances that speed up the rate of the
reaction are called positive catalysts. Materials that slow down the rate of the reaction are
called inhibitors (or negative catalysts). There are some other chemical substances which can
either increase or decrease the activity of catalysts. They are called promoters and catalytic
poisons respectively. Further, catalysts can be divided into two main types known as
heterogeneous and homogeneous catalyst. If the catalyst is in a different phase from the
reactants, then it is called a heterogeneous catalyst. If the catalyst is in the same phase as the
reactants, then it is known as a homogeneous catalyst. As we classified ‘catalysts’ into
subgroups, catalytic processes also can also be classified into two major groups called
homogeneous catalysis and heterogeneous catalysis based on the phase of the catalyst
employed with respect to reactants ( 1).
Heterogeneous catalysis involves the use of a catalyst in a different phase from the
reactants. Typical examples involve a solid catalyst with the reactants as either liquids or
gases. Whereas homogeneous catalysis involves in the use of a catalyst in a same phase as
that o f reactants.
A typical heterogeneous process happens as follows; initially one or more of the reactant
molecules are adsorbed on to the surface of the catalyst at ‘active sites’ during the reaction.
Active sites are the specific sites which are responsible for the activation of the reactant
molecules. In other words, active site is a part of the surface which is particularly good at
adsorbing things and helping them to react. There is some sort of interaction between the
/
(Fhapter J (Sntrodnctien &< literature
surface of the catalyst and the reactant molecules which make them more reactive. This might
involve an actual reaction with the surface, or some weakening o f the bonds in the attached
molecules. At this stage, both o f the reactant molecules might be attached to the surface, or
one might be attached and hit by the other one moving freely in the gas or liquid. Finally, the
product molecules are desorbed from the catalyst surface. Desorption simply means that the
product molecules break away. This leaves the active site available for a new set of molecules
to attach and react.
A good catalyst needs to adsorb the reactant molecules strongly enough for them to react,
but not so strongly that the product molecules stick more or less permanently to the surface.
Silver, for example, isn't a good catalyst because it does not form strong enough attachments
with reactant molecules. Tungsten, on the other hand, isn't a good catalyst because it adsorbs
too strongly. Metals like platinum and nickel make good catalysts because they adsorb
strongly enough to hold and activate the reactants, but not so strongly that the products
cannot break away.
1.2. Heterogeneous catalysis
Heterogeneous catalysis plays a major role worldwide, not only with respect to an
economic viewpoint, but it also provides the necessary infrastructure for the well being of
society as a whole. Without effective heterogeneous catalysis the manufacture of many
materials, pharmaceuticals and foodstuffs would not be possible. It is not surprising,
therefore, that as it is a subject that spans chemistry, chemical engineering and materials
science, there is intense and broad interest in the design of new catalysts as well as seeking to
understand how these materials function as catalysts. With respect to current industrial
processes many of these are based on acid-base catalysts, e.g. catalytic cracking of
2
(PlhTpla 1 introduction &< literature
hydrocarbons for the production o f transportation fuels, and hydrogenations with molecular
hydrogen typically using metal catalysts, e.g. the Fischer-Tropsch synthesis (2)
In addition, selective oxidation remains one of the key synthetic steps for the activation
of a broad range of substrates for the production of either finished products or intermediates
for the preparation o f pharmaceuticals, agrochemicals, as well as commodity chemicals.
However, in contrast to hydrogenation reactions which are often carried catalytically with
molecular hydrogen, there are relatively few selective oxidation reactions that are catalysed
by heterogeneous catalysts using molecular oxygen as an oxygen source. Often selectivity in
these processes can only be achieved if stoichiometric oxygen donors, e.g. manganates, or
activated forms of oxygen, e.g. hydrogen peroxide, are utilised; this increases the costs as
well as significantly decreasing the atom efficiency of the overall process.
While many would consider hydrogenation with a heterogeneous metal catalyst and
molecular hydrogen as a commonplace laboratory procedure, most would not consider doing
a similar oxidation process with molecular oxygen as a standard laboratory procedure. Why is
the case? There is one essential difference between catalytic hydrogenation and oxidation.
Under most reaction conditions, the hydrogen molecule has to be activated by chemisorption
on a catalyst surface before it can be reacted with a substrate for hydrogenation to occur. At
the relatively low temperatures at which catalytic hydrogenation reactions are carried out,
activation of hydrogen via a competing radical pathway is not feasible. This is not the case
with selective oxidation (2 ).
Dioxygen in its ground state is a diradical triplet species. This opens up the possibilities
of competing non-catalysed gas or liquid phase reactions in which triplet dioxygen reacts
directly with the substrate without the intervention of a catalyst. Fortunately, most organic
substrates of interest are in singlet states in their ground states and so this effect can be
minimised. However, it is a competing factor that complicates oxidation reactions, and
5
(‘rlu.Tptej I <^ntwditction &< &Liteiatwe
necessarily makes them far more complex and demanding to study. However, even with this
added complication, given its central importance, it is surprising that there have been few
new approaches in the design o f selective oxidation catalysts using molecular oxygen in the
past forty years (3). Indeed, the major advances in selective oxidation were marked out over
50 years ago; since then there have been the remarkable developments of the titanium
silicalite TS-1 for the epoxidation of alkenes and iron-doped ZSM-5 zeolite for the oxidation
of benzene to phenol. Both of these oxidations are complex and difficult and had previously
eluded many talented researchers; however, both processes use activated forms of oxygen,
namely H2O2 for TS-1 (4) and N2O for Fe-ZSM-5 (5) and both catalysts are totally non-
selective with molecular oxygen. There is, therefore, a real need for new catalytic processes
that use molecular oxygen, especially since environmental factors are of paramount
importance and we need to develop atom efficient green processes. It is against this
background that recent developments in selective oxidation reactions using supported gold
and gold-palladium nanoparticles are beginning to make an impact.
1.3. Gold catalysis
Catalysis using gold nanoparticles is a topic of much current interest. There are several
general reasons that explain this interest. One of them is the fact that the catalytic activity of
gold is directly related to the particle size in the nanometre length scale. Thus, gold catalysis
is a paradigmatic example o f those properties that are only observed in nanoparticles and can
disappear completely as the particle size grows into the micrometric scale. Considering the
current impetus of nanoscience, it is understandable that all the aspects related to the
preparation of small nanoparticles of narrow size distribution, their stabilization and their
unique properties compared to larger particle size are appealing to a large community of
researchers from material science, computational chemistry and catalysis (6 ).
4
(T'hapter / in troduction &C ^ iterative
Since the mid 1990s, there has been a dramatic increase in research attention focussed on the
use of supported gold catalysts for redox reactions. Initial interest in gold catalysts involved
two observations. First, and foremost, Haruta and co-workers (7) identified that nanocrystals
o f Au supported on oxides were highly effective as a catalyst for CO oxidation at subambient
temperature. Second, Hutchings (8) predicted that cationic gold would be the most effective
catalyst for the hydrochlorination of acetylene, and this prediction was subsequently
experimentally verified (9). In the last few years, the number o f publications appearing on
gold catalysis involving both homogeneously and heterogeneously catalysed reactions has
risen exponentially, and these publication statistics have recently been published by Hashmi
(10). With respect to heterogeneously catalysed reactions, most o f the publications concern
the oxidation o f CO at ambient temperature, and these data have been reviewed by Bond and
Thompson (11,12) and by Haruta (13). Much less attention has focussed on selective
oxidation, and yet supported gold catalysts have been found to be very effective for a broad
range o f selective oxidation reactions. Haruta and co-workers (14) and Moulijn and co-
workers (15) have shown that Au supported on microporous or mesoporous titanium silicate
structures is very effective for the oxidation of propene to propene oxide with O2/H2
mixtures, and there is currently significant industrial interest in this reaction. The seminal
studies of Rossi and Prati (16-19) have shown that supported Au catalysts are effective for
the oxidation of alcohols in the liquid phase with O2 to form monoacids when a base is
present. Biel la and Rossi (20) extended this to show that the same catalysts can be used with
gas phase reactants and aldehydes could be formed in high selectivity. Subsequently,
Hutchings et al showed (21, 22) that Au/C catalysts could be very selective for glycerol
oxidation to glycerate. More recently, P.Landon et al have shown that AU/AI2O3 catalysts are
selective for the oxidation o f H2 with molecular oxygen to form hydrogen peroxide (23, 24).
Chaplet / introduction &< lite r ature
These examples demonstrate the immense potential of gold catalysts for selective oxidation
reactions (25).
1.4. Oxidation of toluene
Selective oxidation o f primary carbon-hydrogen bonds is of crucial importance in
activating raw materials to form intermediates and final products for use in the chemical,
pharmaceutical and agricultural business sectors (26). One class of raw materials is alkyl
aromatics; toluene, for example, the simplest member of this class, can be oxidized to benzyl
alcohol, benzaldehyde, benzoic acid, and benzyl benzoate as shown in scheme 1.1. These
products are commercially significant as versatile intermediates in the manufacture of
pharmaceuticals, dyes, solvents, perfumes, plasticizers, dyestuffs, preservatives, and flame
retardants.
CHO COOH
CH. +
/ r i .
o
O CH-
Scheme 1.1. General reaction pathways for toluene oxidation.
W.Partenheimer has reported that benzyl alcohol and benzaldehyde can be produced by
the chlorination of toluene followed by hydrolysis/oxidative hydrolysis in some cases (27).
Chapter 1 ^ntrochiction £< ^literature
However the chlorination method has its own disadvantage of not conforming to the quality
of food chemical codex (F.C.C), relatively high production cost, and large amount of
pollutant residues. The major demand for benzaldehdye is being met by the dedicated benzoic
acid production plants wherein the benzaldehyde is formed as a by-product in small
proportions in air oxidation of toluene at higher conversion. As the process of benzoic acid
from phenol is currently uneconomical, the benzoic acid produced through this process is
unable to find good markets.
Benzaldeyde and Benzoic acid is produced by the liquid-phase cobalt-catalyzed reaction of
toluene using oxygen at 165°C with acetic acid as solvent, but the conversion has to be
limited to <15% to retain high selectivities (28-34). The use of halogens and acidic solvents
makes these processes environmentally unfriendly.
(A)
Figure 1.1. (A) Polyhedral representation of the Keggin-type POM, [01-PW12O40]3 , where one PO4 group is shown as the internal, black tetrahedron. (B) Polyhedral representation of the Dawson-type POM, [a-P2 W|g062]6, where two PO4 groups are shown as the internal, gray tetrahedron. (29)
Nomiya et al (29) have reported vanadium (V) - substituted polyoxometalates
catalyzed toluene oxidation using 30% H20 2. This process lacks true synthetic value because
7
(plkrpter I introduction cBt ^Literature
of the relatively low yields (0.2-0.9% conversion). Vanadium (V) - substituted
polyoxometalates used for this process are shown in figure 1.1.
T.G.Carrell et al (30) has reported the oxidation of a variety of substrates (thioethers,
hydrocarbons, alkenes, benyzl alcohol and benzaldehyde) by /-BuOOH catalyzed by
Mn404(0 2PPh2)6 ( 1) and Mn4O4(O2P09-MePh)2)6 (2 ) as shown in figure 1.2 .
\
0 2PPh2 (in 1)
0 2P(p-MePh)2 (in 2)
Figure 1.2. Molecular representation of the Manganese oxo cubane catalysts. Catalysts 1 and 2 differ only by a methyl group in the /rara-position of the phenyl moiety in the diphenylphosphinate ligand (30)
Saravanamurugan and co-workers (31) have done oxyfunctionalisation of toluene with
activated /-butyl hydroperoxide (TBHP) in the liquid phase over Ti(IV) supported on NaY
zeolite (Ti-NaY), silylated Ti-NaY (Sil-Ti NaY), silica-supported titanium isopropoxide (Si-
Chapter I Q&ntroductwn &< ^Literature
Ti(OiPr) and silylated silica-supported Ti(OiPr) /Sil-Si-Ti(OiPr)/at 30 and 80 °C. The major
products obtained with this method were benzaldehyde and benzoic acid. They found that
Silylation o f catalysts favours high conversion o f TBHP, thus proving that a hydrophobic
environment around the active Ti(IV) sites is important for selective adsorption of TBHP.
Liquid-phase oxidation of toluene by air in acetic acid medium, with cobalt acetate
catalyst and sodium bromide or paraldehyde as promoter, has been investigated by
H.V.Borgaonker and co-workers (32) with a view to developing a process for the production
o f benzaldehyde with benzoic acid as a coproduct. Up to 10% conversion, the use of either
sodium bromide or paraldehyde as a promoter gave more than 90% yield of benzaldehyde. At
higher conversions, the yield o f benzaldehyde decreased markedly and benzoic acid was
obtained as a co-product. Under suitable conditions it was possible to get 40% yield of
benzaldehyde, provided the overall conversion of toluene was restricted to 20%. Kantam et al
(33) have studied the liquid phase oxidation of toluene using Co/Mn/Br' system in order to
produce benzyl alcohol and benzaldehyde from toluene. But the use o f bromide as promotor
makes this process undesirable due to environmental and safety issues. Further, in bulk
chemical manufacture, traditional environmentally unacceptable processes are largely being
replaced by cleaner catalytic alternatives.
Attempts have been made to achieve for gas phase oxidation of toluene (35). Many of the
catalysts described are based on molybdenum in combination with tungsten (36), manganese
(37), cerium (38), or iron (39) in the form of mixed oxides or based on carriers such as
molecular sieves (40, 41). One of the most promising gas phase catalysts for benzaldehyde
production, free of molybdenum, is based on V^Os-supported on Ti0 2 , Si0 2 , or AI2O3 (42-
45). The catalytic activity of these catalysts could be increased in some cases with additives
such as K2SO4 (45). But these catalysts are not economically viable as they have got low
surface area.
9
(Phapter 1 (S-ntreductien cSt ^Literature
F.Konietzni et al have reported that toluene can be oxidized into benzaldehyde by an
amorphous, microporous Mn/Si mixed oxide (46) under vapour phase conditions. After
determining reaction conditions that exclude mass transport and pore diffusion limitations,
selectivitity of 83% were achieved, but at a total conversion of 4%. The conversion profiles
with respect to flow and particle size are shown in figure 1.3 and 1.4.
6 -
o 4 —
1 0 08040 6020Flow [mi/min]
Figure 1.3. Vapour phase oxidation o f toluene using Mn/Si mixed oxide (46)
10
Chapter / introduction Sc £ literature
■ conversion [GC-%] □ yield [GC-%]
20-50 > 100ballmill 1-2 pm
particle s ize
Figure 1.4. Vapour phase oxidation of toluene using Mn/Si mixed oxide. Conversion Vs particle size of Mn/Si mixed oxide (46)
Hence vapor-phase oxidation of toluene has the disadvantage that conversion must be
limited to avoid over oxidation to CO2 and other by-products (46). Attempts to overcome
these problems have prompted the investigation of the use of supercritical CO2 and ionic
liquids. J.Zhu and co-workers (47) have studied toluene oxidation using aqueous emulsion
containing ionic Co2+ and Br2 species stabilised by fluorous surfactant-like species in
supercritical CC>2-air mixture. Unfortunately the conversion was low. Further, Seddon and
Stark (48) have reported the use of ionic liquid as a reaction media. Though this methodology
works well for benzyl alcohol oxidation, it failed to work for toluene oxidation resulting very
low conversion.
/ /
Chapter / introduction S t ^literature
Often, heterogeneous catalysts are preferred over homogeneous catalysts, because these
materials can be readily separated from the reaction mixture. Heterogeneous catalysts can
also be readily used in flow reactors, facilitating the efficient production of materials using
continuous processes. Another important aspect is carrying out the reactions under solvent
free conditions. The use of a heterogeneous catalyst in the solvent-free condition has the
advantages of minimizing separation difficulty and equipment corrosion.
For the oxidation of toluene, there have been many attempts to find a suitable oxidation
catalyst, and to date these have used copper and manganese (49-51), cobalt (52), or
chromium (53) catalysts. X.Li and co-workers (49) have studied toluene oxidation using
molecular oxygen as an oxidant under solvent free conditions in the presence of Cu-Mn
mixed oxides. The percentage conversion values of various bimetallic mixed oxides are
shown in figure 1.5.
Cu,Mn Cu.Zn Cu.Si Cu,Zr Cu,Cr Co.Mn Ce.Mn Fe.Mn
Catalysts
Figure 1.5. Conversion of toluene using copper and manganese based catalysts at 190 °C (49)
12
(Chapter I ^ntw ductw i £< ^Literature
F.Wang et al (50) has reported that the liquid phase oxidation of toluene with molecular
oxygen over heterogeneous catalysts o f copper-based binary metal oxides. Among the
copper-based binary metal oxides, iron-copper binary oxide (Fe/Cu=0.3 atomic ratio) was
found to be the best catalyst in their process yielding 7% toluene conversion.
J.Gao et al (51) have reported MnCC>3 promoted toluene oxidation at 190 °C, in which
MnCC>3 can readily form high-valence Mn species in the presence of oxygen and that this
high-valence Mn species activates aromatic hydrocarbons to the corresponding radicals.
R.L.Brutchy and co-workers (52) have studied the oxidation of alkyl aromatics using a
pseudo-tetrahedral Co (II) complex grafted onto the surface of SBA-15 as shown in scheme
1.2, by utilizing tert-butyl hydroperoxide (TBHP) via an H-atom transfer mechanism.
OH OH
silica
(di-rBu*bipy)Co[OSi(Ofeu)3l2
silica
Scheme 1.2. A typical formation of Co (II) complex supported on SBA-15 (52)
A.P.Singh and T.Selvam (53) have reported that oxidation of toluene and alkanes
catalyzed by chromium silicalite-1 (CrS-1) using t-butyl hydroperoxide (TBHP) as an
oxidant. Mechanistically, it is assumed that catalytic properties of CrS-1 originate from the
reversible transformation of Cr3+ and Cr5+ within the structure of CrS-1. But the use of
chromium makes this process unsuitable as the chromium is toxic.
13
Chapter / introduction &Z ^Literature
All these catalysts mentioned above perform very poorly with turnover numbers (TONs:
mole o f product per mole of metal catalyst) of less than 100, even at temperatures in excess
o f 190°C (Table 1.1).
Table 1.1: Reported toluene oxidation system
Selectivity (%)
Catalyst T/P OxidantConv
(%)
Benzyl
alcohol
Benzal
dehyde
Benzoic
acid
Benzyl
benzoateTON
Cu-Mn (1/1)190 °C
IMPa0 2 21.6 1.6 9.2 73.7 13.6 8
Cu-Fe/y-
A120 3
190 °C
IMPa0 2 25.4 1.0 27.4 71.6 n.d. 74
M nC03190 °C
IMPaO2 25.0 5.3 9.7 80.8 n.d. 50
CoSBA-1580 °C
1 atmTBHP 8.0 n.d. 64.0 n.d. n.d. 103
Cr/Silicalite80 °C
1 atmTBHP 18.4 5.2 23.3 25.7 n.d. n.d.
There is clearly a need to develop heterogeneous catalysts for toluene oxidation that have
greatly improved activity while retaining selectivity. Recently, it has been shown that Au-Pd
alloy nanoparticles are very effective for the direct synthesis of hydrogen peroxide (54) and
the oxidation of primary alcohols using oxygen (55). J.K.Edwards et al (54) have synthesized
sub 10 nm Au-Pd nanoparticles (prepared by impregnation), which have shown that very
active for the synthesis o f hydrogen peroxide, directly from oxygen and hydrogen (Scheme.
1.3).
14
Chapter / introduction S t ^Literature
Scheme 1.3. Direct synthesis o f hydrogen peroxide (54)
Further, Enache and co-workers (55) studied the oxidation of primary alcohols using
oxygen (figure 1.6) in place o f stoichiometric oxygen donors under solvent free conditions in
the presence o f AuPd core shell nanoparticles (Pd-shell, Au-core) prepared by impregnation.
These catalysts have shown high turnover frequencies (86 500 - 270 000 turnovers per hour)
for benzyl alcohol (relative o f toluene) and 1-phenylethanol at 160 °C.
A
0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)
Figure 1.6. Benzyl alcohol conversion and selectivity in benzaldehyde with the reaction time at 373 K and 0.1 MPa pC>2. Squares, Au/Ti0 2 ; circles, Pd/TiC>2; and triangles, Au-Pd/Ti0 2 - Solid symbols indicate conversion, and open symbols indicate selectivity (55)
The catalytic data show that the introduction of Au to Pd improves selectivity, and they
believed that the surface o f the bimetallic nanoparticles will still contain some gold. Hence,
IS
Chapter / (Zintroduction S t literature
they argued that the Au acts as an electronic promoter for Pd and that the active catalyst has a
surface that is significantly enriched in Pd. Recent studies have started to provide insights
into the nature o f such effects. For example, Okazaki et al. (56) have shown, using a
combination of experiment and theory that the electronic structure of Au in Au/Ti02 catalysts
is dependent on the particle size and Goodman and co-workers (57), using model studies,
have shown that Au can isolate Pd sites within bimetallic systems.
This catalyst operates by establishing a reactive hydroperoxy intermediate as these
intermediates are known to be involved in the enzymatic oxidation of primary carbon-
hydrogen bonds (58). Colby et al (58) have found that Methane mono-oxygenase of
Methylococcus capsulatus (Bath) catalyses the hydroxylation of C1-C8 n-alkanes, yielding
mixtures o f the corresponding 1- and 2-alcohols via reactive hydroperoxy intermediate. This
indicates that both primary and secondary alkyl C-H bonds can be hydroxylated. The enzyme
is apparently specific for the 1- and 2-alkyl carbon atoms, however, as there was negligible
formation o f 3- or 4-alcohols from pentane, hexane, heptane or octane.
Colby et al also tested toluene as a potential enzyme substrate because it is the phenyl
derivative o f methane and the methyl derivative o f benzene and therefore contains two
different groups that could be hydroxylated. A mixture of benzyl alcohol and cresol was
produced when the enzyme was incubated with toluene, indicating that both the aromatic ring
and the methyl group were hydroxylated (58). Hence, it is reasoned that it should be feasible
for Au-Pd nanoparticles to be active for the oxidation of the primary carbon-hydrogen bonds
in toluene too. The present works conveys that Au-Pd alloy nanoparticles prepared by a sol
immobilization technique can give significantly improved activity for the oxidation of
toluene under mild solvent-free conditions.
16
Chapter 1 introduction S t £ literature
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1. Heterogeneous Catalysis: Principles and Applications, G.C. Bond, Oxford chemistry
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2. Graham J. Hutchings, Chem. Commun., 1148-1164 (2008)
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15307-15313 (2009)
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8 . G.J. Hutchings, J. Catal, 96, 292 (1985)
9. B. Nkosi, N.J. Coville, G.J. Hutchings, M.D. Adams, J. Friedl and F. Wagner, J. Catal.
128, 366(1991)
10. A.S.K. Hashmi, Gold Bull. 37, 51 (2004)
11. G.C. Bond and D.T. Thompson, Catal. Rev.-Sci. Eng. 41, 319 (1999)
12. G.C. Bond and D.T. Thompson, Gold Bull. 33, 41 (2000)
13. M. Haruta, Gold Bull. 37, 27 (2004)
14. T. Hayashi, K. Tanaka and M. Haruta, J. Catal. 178, 566 (1998)
15. T.A. Nijhuis, B.J. Huizinga, M. Makkee and J.A. Moulijn, Ind. Eng. Chem. Res. 38,
884(1999)
16. L. Prati and M. Rossi, J. Catal. 176, 552 (1998)
17. F. Porta, L. Prati, M. Rossi, S. Colluccia and G. Martra, Catal. Today, 61, 165 (2000)
18. C. Bianchi, F. Porta, L. Prati and M. Rossi, Top. Catal. 13, 231 (2000)
19. L. Prati, Gold Bull. 32, 96 (1999)
20. S. Biella and M. Rossi, Chem. Comm. 378 (2003)
21. S. Carrettin, P. McMom, P. Johnston, K. Griffin and G.J. Hutchings, Chem. Commun.
696 (2002)
22. S. Carretin, P. McMom, P. Johnston, K. Griffin, C.J. Kiely and G.J. Hutchings, Phys.
Chem. Chem. Phys. 5, 1329 (2003)
23. P. Landon, P.J. Collier, A.J. Papworth, C.J. Kiely and G.J.Hutchings, Chem. Commun.
2058, (2002)
24. P. Landon, P.J. Collier, A.F. Carley, D. Chadwick, A.J.Papworth, A. Burrows, C.J.
Kiely and G.J. Hutchings, Phys.Chem. Chem. Phys. 5, 1917 (2003)
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Chapter / introduction S t ^Literature
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Chapter / introduction &<. ^literature
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19
Chapter 2(E^perimenta[
Chapter 2 Experimented
2.1. Outline
This chapter discusses the experimental methodology employed in this research project
which is ‘’Solvent-free oxidation of primary carbon-hydrogen bonds in toluene using
supported gold palladium alloy nanoparticles” . In this section, various preparation protocols
of catalyst materials including impregnation, sol immobilisation are discussed. Further,
instrumental methods for miscellaneous characterizations of catalysts, application of these
catalysts for the oxidation o f desired substrates, qualitative and quantitative analysis of
products are detailed.
2.2. Catalyst Preparation
2.2.1. Impregnation method
2.5wt%Au-2.5wt%/TiC>2 catalyst was prepared by impregnation method. An aqueous solution
of FLAUCI4 .3 H2O (Johnson Matthey) (4.0816 ml, which is pipetted out from the stock solution
containing 12.25 mg/ml Au) and an aqueous solution of PdCh (Johnson Matthey) (16.51 ml,
which is pipetted out from the stock solution containing 3.0287 mg/ml), 1.9 g o f TiC>2 were added
in a clean, empty 25 ml beaker. The beaker was placed on a heating plate with magnetic stirrer and
started to stir and heat (temperature set point 90°C). Initially the solution was milk white. When
the temperature increased, the colour of the solution turned to purple slowly. Once the water in the
beaker evaporated out, the beaker was taken out from the heating plate and it was covered with
aluminium foil and kept in oven for drying at 110°C for 16 hours and the small portion of the dried
catalyst (typical 0.5 g) calcined at 400°C for 3 hours under static air condition. Similarly 2.5%Au-
only and 2.5%Pd-only were prepared.
2.2.2. Sol immobilisation method (SI)
Au-Pd monometallic and bimetallic sols were prepared using the following procedure: An
aqueous PdCh and HAuCU solution o f the desired concentration was prepared individually (for
20
Chapter 2 <3Lxperimefital
monometallic) as well as together (for bimetallic). The desired amount of a Polyvinyl alcohol
(PVA) (Aldrich, MW=10 000, 80% hydrolyzed) 1 wt % solution was added (PVA/Total metals
(wt/wt) = 1.2); a 0.1 M freshly prepared solution of NaBH4 (Aldrich) (NaBH4/Total metals
(mol/mol) = 5) was then added to form a dark-brown sol. After 30 minutes of sol generation,
the colloid was immobilised by adding activated carbon (or titania) (acidified to pH 1, by
sulphuric acid) under vigorous stirring. The amount o f support was calculated as having a total
final metal loading of 1% wt. After 2 h the slurry was filtered, the catalyst washed thoroughly
with distilled water (neutral mother liquors) and dried at 110 °C for 16 hours.
2.3. Characterization of catalysts
2.3.1. UV- Visible spectroscopy
The colloidal sols (before immobilising on a support) were characterised using a UV
spectrometer (V-570, JASCO) in H2O between 200 and 900 nm, in a quartz cuvette to monitor
the intensity and position o f the plasmon resonance band o f metal nanoparticles.
For the pure Au sol, the appearance o f the plasmon resonance was observed at 505 nm.
This resonance peak is characteristic for gold nanoparticles with particle sizes below 10 nm (1,
2). In the case o f the Pd monometallic sol, there is no surface plasmon band (1). The spectra
resulting from the mixture o f the two metal sols indicate the disappearance of the gold surface
plasmon band as previously reported for this system (1) (figure 2.1). This is a phenomenon
commonly found in the formation of sols of bimetallic nanoparticles where one of the
component metals lacks a surface plasmon band. Deki et al (3) found dispersed nanoparticles
of several Au-Pd compositions on a polymer thin film matrix and concluded that the
progressive decrease o f the gold plasmon band in the UV-vis spectra observed by increasing
the palladium content was the result of changes in the band structure o f the Au particles due to
alloying with Pd.
21
Chapter 2 GLxperimertlai
Um.oo«•
200 300 400 500 700600 800 900
Wavtltnglh (nm)
Figure 2.1. A typical UV-vis spectra of Au-Pd (0) sol showing no plasmon resonance band (4)
2.3.2. X-ray photoelectron spectroscopy
X-ray photoelectron spectra were recorded on a Kratos Axis Ultra DLD spectrometer
employing a monochromatic Al Ka X-ray source (75-150W) and analyzer pass energy of 160
eV (for survey scans) or 40 eV (for detailed scans). Samples were mounted using double-sided
adhesive tape, and binding energies were referenced to the C(ls) binding energy of
adventitious carbon contamination which was taken to be 284.7 eV.
2.3.3. Transmission electron microscopy (TEM/STEM)
Samples of sol-immobilized catalysts were prepared for TEM/STEM analysis by dry
dispersing the catalyst powder onto a holey carbon TEM grid. In the case o f the starting AuPd-
PVA sol, a drop o f the colloidal sol was deposited, and then allowed to evaporate, onto a 300-
mesh copper TEM grid covered with an ultrathin continuous C film. Phase contrast lattice
imaging and high-angle annular dark field (HAADF) imaging experiments carried out using a
200kV JEOL 2200FS transmission electron microscope equipped with a CEOS aberration
corrector kV in order to study the structural and compositional details o f individual metal
nanoparticles . This instrument was also fitted with an Oxford Instruments INCA TEM 300
22
Chapter 2 GLzpaimaitai
system for energy dispersive X-ray (XEDS) analysis in order to determine particle size
distributions and compositions.
2.4. Catalyst testing
2.4.1. Toluene oxidation
The toluene oxidation reactions were carried out in a stirred reactor (100 mL, Parr reactor).
The vessel was charged with the desired substrate (10-40 mL) and catalyst (0.2-1.6 g). The
autoclave was then purged 5 times with oxygen leaving the vessel at 150 psi pressure. The
stirrer was set at 1500 rpm and the reaction mixture was raised to the required temperature.
Samples from the reactor were taken periodically, via a sampling system.
2.4.2. Catalyst reuse studies
Decantation method
After the completion o f the reaction, the mixture was centrifuged and the liquid above the
solid catalyst was completely decanted. A fresh solution of the required volume of toluene was
added and the reaction was repeated. The decanted liquid was analysed for leached metals
using atomic absorption spectroscopy and inductively coupled plasma spectroscopy and metal
concentrations were below the detection limits.
2.4.3. Benzyl alcohol oxidation
The benzyl alcohol oxidation reactions were carried out in a stirred reactor (Parr autoclave, 100
ml). The cleaned empty reactor vessel was charged with 40 ml of benzyl alcohol and 50 mg o f a
catalyst. The autoclave was purged 5 times with oxygen and the pressure inside the vessel was
adjusted to 10 bar gauge. The stirrer was set at 1500 r.p.m. and the reaction mixture was heated at
23
S ta p ler 2 (3ucperim ental
the desired temperature. Samples from the reactor were taken out periodically through the
sampling tube.
2.4.4. Analysis of products
For the identification and analysis o f the products a GC-MS and GC (a Varian star 3400 cx
with a 30 m CP-Wax 52 CB column) were employed. In addition the products were identified
by comparison with known commercially pure samples. For the quantification of the amounts
of reactants consumed and products generated, the external calibration method was used.
2.4.5. Toluene oxidation using H2O2
The toluene oxidation using H2O2 experiments were carried out in a stainless steel
autoclave containing a Teflon liner vessel with a total volume o f 50 ml. The hydrogen peroxide
(50 %, 0.34 g, 5000 pmol) was weighed in a 10 ml standard flask and then the flask was made
up to the mark with an aqueous solution of toluene (4.34x10'3 M, 10 ml, 43.4 mmol). Then the
whole homogeneous solution was transferred to a reactor vessel containing a measured amount
of catalyst, corresponding to 10'5 moles o f total metal. The reactor was purged with helium
with the pressure o f 440 psi for 5 times, to get rid of all the atmospheric air from the system.
The autoclave was heated to the desired reaction temperature (50°C), and the reaction mixture
was vigorously stirred at 1500 rpm once the target temperature was reached. The reactions
were carried out for a total o f 0.5 h, after which the reactor was cooled in ice to below 12°C, in
order to minimise the volatility o f substrate as well as products. The resultant mixture was
filtered and analysed with a combination of GC-MS and GC-FID.
24
Chapter 2 GLxperimentai
References:
1. Bianchi, C. L.; Canton, P.; Dimitratos, N.; Porta, F.; Prati, L. Catal. Today 2005, 102-103,
203-212.
2. Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103,4212^217.
3. S. Deki, K. Akamatsu, Y. Hatakenaka, M. Mizuhata and A. Kajinami, Nanostructured
Mat. 11 (1999) 59.
4. James Pritchard, Lokesh Kesavan, Marco Piccinini, Qian He, Ramchandra Tiruvalam,
Nikolaos Dimitratos, Jose A. Lopez-Sanchez, Albert F. Carley, Jennifer K. Edwards,
Christopher J. Kiely, Graham J. Hutchings, Langmuir 2010, 26 (21), 16568-16577
2S
Chapter 3SoCvent-Tree Oxidation ofToCuene
Chapter 3 Q&ohwili freo (Oxidation of rColiteno
3.1. Outline
This chapter deals with the reaction optimisation of toluene oxidation under solvent free
conditions. The effect o f temperature, choice of supports, selection of catalyst preparation
method, studying the effect o f Au:Pd mol ratio in bimetallic Au-Pd sol catalysts, and attempts
made to increase the conversion of toluene has been studied. These studies helped to screen
the active catalyst and optimum reaction conditions to obtain high conversion of toluene.
Further, this chapter discusses the results obtained from longer reactions of toluene oxidation
with increased mass o f catalyst, structure - activity relationship of the Au-Pd bimetallic sol
catalysts, and results gained through low temperature oxidation o f toluene. The catalysts are
prepared by either impregnation or sol immobilisation methods. Most of the catalysts
prepared uses activated carbon as the support although a few comparisons were made with
TiC>2 . Molecular oxygen is used as an oxidant for all the reactions throughout these studies.
No solvent has been used throughout the studies.
3.2. Reaction optimization
3.2.1. Blank toluene oxidation
Toluene oxidation has been carried out in an autoclave parr reactor as described in
chapter 2. As a first step, toluene oxidation has been performed with oxygen in the absence of
a catalyst at elevated temperature ca. 160 - 190 °C in order to determine the blank baseline
rate. The reason for choosing this temperature range is that many o f the workers (1-3) have
suggested activity for toluene in this window. A considerable conversion o f toluene into its
oxygenates was observed at 190 °C, but not at 160 °C as shown in figure 3.1 and table 3.1
respectively. The products which we identified from these reactions are benzyl alcohol,
benzldehyde, benzoic acid and benzyl benzoate. There was no CO2 formed.
26
Chapter 3 Qffohvnt-i ree (Qridation o frCohiem
r 70
- 60
■ 50
■4 0 i*
- 30 JS<D(0
- 20
- 10
0
Figure 3.1. Toluene conversion and selectivity to benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate in the absence of a catalyst at 190 °C. Reaction conditions: T = 190 °C (463K), / 7O2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml toluene, 48 h. Open symbol indicates conversion and solid symbols indicate selectivity. Key: ■ indicate selectivity to benzyl alcohol, ♦ indicate selectivity to benzaldehyde, A indicate selectivity to benzoic acid, • indicate selectivity to benzyl benzoate.
The blank toluene oxidation at 160 °C gave conversion of 0.2% after 7 h, whereas the
experiment carried out at 190 °C yielded conversion of 0.9%. But at 48 h, the conversion drift
was 2.9% and 18% respectively. This can be due to the involvement o f O2 di-radical which
can initiate homogeneous oxidation processes at elevated temperatures and pressures. It has
been found that such processes become substantial (18%) at 190 °C under our reaction
conditions. In view o f the potential role of O2 di-radicals, the maximum reaction temperature
in our studies has been restricted to 160 °C as the blank conversion at shorter reaction period
was negligible and was very low at longer reaction times at this temperature.
20 -1
18 -
16 -
14 -c0•g 1 2 -a>1 10 -oo® 8 -4)3OI-
0 10 40Time (h)
27
Chapter 3 Qfbobent-t free Oxidation o f r£oluono
Table 3.1: Liquid phase oxidation of toluene in the absence of catalyst at 160 °C
CatalystTime Conv Selectivity (%)
TON(h) (%) Benzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoate
None 7 0.2 18.3 69.4 1.9 10.4 -
None 48 2.9 5.8 43.6 38.2 12.4 -
Reaction conditions: T = 160 °C (433K), p 0 2 = 10 bar (0.1 MPa), stirring rate - 500 rpm,substrate - 20 ml toluene
3.2.2. Toluene oxidation in the presence of catalyst supports
Further toluene oxidation experiments have been performed with the chosen catalytic
supports which are carbon and Ti(> 2 at 160 °C for 7 h to check their contribution of activity in
this system. The reason for choosing carbon and TiC>2 as catalytic supports is due to their high
surface area (-900 m2g 1) and less acidic nature respectively. Moreover, these two supports
have shown the ability to adsorb metal nanoparticles effectively followed by a homogeneous
distribution and to reduce the by product formation during the course of the reaction (4, 5).
The blank reaction in the absence of catalyst but in the presence o f support is negligible for
short reaction times (Table 3.2). This has allowed using these materials as catalytic supports.
Table 3.2: Liquid phase oxidation of toluene in the absence of catalyst, but in the
presence o f support at 160 °C
SupportTime
(h)
Conv
(%)
Selectivity (%)TONBenzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoate
Carbon* 7 0.1 11.4 72.6 0.1 16.0 -
T i02* 7 0.2 0.1 6.0 85.1 8.9 -
Reaction conditions: T = 160 °C (433K), p 0 2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 20 ml toluene, support - 0.4 g.
28
Chapter 3 Qtbofoent freo (Eteidatwn o f rColuene
3.2.3. Toluene oxidation using mono and bimetallic Au Pd supported catalysts prepared
by impregnation
To study the influence o f catalysts on toluene oxidation, monometallic Au and Pd,
bimetallic Au Pd supported on TiC>2 prepared by impregnation were tested initially, because
these had previously been shown to be very active for H2O2 synthesis (6) and alcohol
oxidation (5). The preparation o f catalysts by the impregnation method was detailed in
chapter 2. The reaction was performed in an autoclave parr reactor as described in chapter 2.
The reaction conditions are as follows; T = 160 °C (433K), p O 2 = 10 bar (0.1 MPa), stirring
rate - 1500 rpm, substrate - 40 ml toluene, substrate/metal molar ratio - 5200, t = 48 h. The
catalyst amount used in this study has been adjusted for every reaction in order to maintain
the substrate to metal molar ratio constant. It has been observed that monometallic Au/Ti02
having 2.5wt% Au was not at all active for this reaction, whereas monometallic 2.5%Pd/Ti02
and bimetallic 2.5%Au2.5%Pd/TiC>2 gave 3.5% and 4.4% conversion after 48 hrs respectively
(Table 3.3). The turnover numbers with these catalysts were poor. Thus, the Au, Pd
monometallic and bimetallic catalysts prepared by the simple impregnation method were not
found to be particularly active for toluene oxidation, although they did not produce any CO2 .
29
&hapler 3 Q&obenti free <£>sddatton e>frCe>kiene
Table 3.3: Liquid phase oxidation of toluene at 160 °C for mono and bimetallic
supported catalysts prepared by impregnation method.
CatalystTime
(h)
Conv
(%)
Selectivity (%)
Benzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoate
TON
2.5%AuTi02 0.5 0.2 89.9 10.1 0.0 0.0
1 0.2 91.4 8.6 0.0 0.0
7 0.2 78.0 13.7 8.3 0.0
24 0.3 70.4 20.3 5.4 3.9
31 0.3 47.1 35.5 7.4 10.0
48 0.5 20.0 53.1 5.8 21.2 23
2.5%Au2.5%Pd/Ti02 0.5 0.1 52.3 32.8 9.5 5.3
1 0.1 31.3 56.9 2.5 9.3
7 0.4 14.9 63.4 12.7 9.0
24 3.0 5.3 43.4 31.3 20.0
31 3.5 4.7 41.4 32.1 21.8
48 4.4 3.6 31.4 40.4 24.6 174
2.5%Pd/Ti02 0.5 0.1 30.6 44.1 23.8 1.5
1 0.1 6.8 47.5 39.0 6.7
7 0.6 7.7 66.8 16.5 9.1
24 2.7 5.7 50.7 30.4 13.2
31 2.9 4.8 47.6 33.8 13.8
48 3.5 3.2 37.4 43.3 16.1 185
Reaction conditions: T = 160 °C (433K), p O i = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml toluene, substrate/metal molar ratio = 5200, TON (moles of products/moles of metal) at 48 h reaction. TON numbers were calculated on the basis of total loading of metals.
30
Chapter 2 Q2bobent- free (Oxidation o f 'Coluene
This can be understood by the fact that Au-Pd nanoparticles synthesized by the
impregnation method can be relatively large (typically >6 nm) and have substantial
compositional variations (Au core-Pd shell) (5, 7) (figure 3.2), which limits their reactivity.
Figure 3.2. A reconstructed MSA-filtered Au-Pd-Ti composition map o f a single particle
(map - Ti, red; Au, blue; and Pd, green) for the impregnated 2.5wt%Au-2.5wt%Pd/Ti02 (5)
21
tChapter 3 Q&obetU free (Oxidation of 0'vluene
3.2.4. Toluene oxidation using mono and bimetallic Au Pd supported catalysts
prepared by sol immobilisation
In order to design more effective catalysts for toluene oxidation, supported Au-Pd
nanoparticles with smaller median particle sizes and a more controlled composition have
been prepared and investigated. These were synthesized by sol immobilization of Au-Pd
colloids, with carbon and TiC>2 as supports, a method that afforded tightly controlled particle
size distributions and composition. Initially, bimetallic AuPd/C and AuPd/Ti02 having a total
metal loading lwt% (the mol ratio of Au:Pd was 1:1.85 or often called as 1:1 wt ratio of
Au:Pd, i.e . 0.5wt%Au, 0.5wt%Pd) catalysts were tested for toluene oxidation at 160 °C. The
conversion was 4.8% and 2.1% respectively, after 7 hrs (Table 3.4), which is a promising
result when compared with the literature (1-3).
Table 3.4: Liquid phase oxidation of toluene at 160 °C using the catalysts prepared by
sol immobilisation method.
Selectivity (%)
Catalyst Time Conv Benzyl Benzaldehyde Benzoic Benzyl TON
(h) (%) alcohol acid benzoate
1 % AuPd/C
(1:1 wt or 7 4.8 0.9 12.7 10.3 76.1 310
1:1.85 mol)
1 %AuPd/Ti02
(l:lw t or 7 2.1 2.9 6.6 1.0 89.5 135
1:1.85 mol)
Reaction conditions: T = 160 °C (433K), pO i = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 20 ml toluene, catalyst - 0.4 g, substrate/metal molar ratio = 6500, TON (moles of products/moles o f metal) at 7 h reaction. TON numbers were calculated on the basis of total loading of metals.
32
Chapter 3 Q$k>bent <free (Oxidation o frColuene
This encouraging result lead to the search for the optimal Au:Pd ratio for toluene
oxidation by keeping lwt% total metal on the catalysts. Having shown double the activity of
Au-Pd/TiC>2, the Au-Pd/C has been chosen for further studies throughout this work.
3.2.4.I. Studying the effect of Au:Pd molar ratio of Au-Pd bimetallic sol catalysts upon
toluene oxidation
A systematic set of carbon supported Au-Pd colloidal nanoparticles having a range of
Au/Pd ratios (by keeping lwt% total metal) prepared by sol immobilisation were tested for
toluene oxidation at 160 °C for 7 hrs (Table 3.5). The molar ratio o f substrate to metal was
kept constant as 6500 for all the reactions in this section. For all catalysts, no CO2 formation
was observed; the only products were benzyl alcohol, benzaldehyde, benzoic acid, and benzyl
benzoate. By itself, Au was not active for toluene oxidation, but the addition of Pd
significantly enhanced the conversion, demonstrating a clear synergistic effect for the Au-Pd
catalysts as compared with the monometallic species. The catalysts have been structurally
characterized using a combination o f UV-visible spectroscopy, transmission electron
microscopy, STEM HAADF/XEDS, and X-ray photoelectron spectroscopy (See Thesis
section 5.2.1). Characterizations o f these catalysts are well detailed in associated with benzyl
alcohol oxidation using the same set o f catalysts. The Au-Pd nanoparticles are found in the
majority o f cases to be homogeneous alloys, although some variation is observed in the AuPd
composition at high Pd/Au ratios.
Also, a physical mixture o f the separate Au/C and Pd/C catalysts showed no
enhancement (Table 3.5, last entry), highlighting the molecular-scale nature o f the synergy.
The observed synergy is due to electronic and morphological features because transmission
electron microscopy (TEM) shows that the mean particle size o f the nanoparticles decreases
slightly on the addition o f Au to Pd (Table 3.6).
33
(Chapter 3 Qfoohwit free (Oxidation o frColnene
Table 3.5: Effect o f Au:Pd mol ratio studied for liquid phase oxidation of toluene at 160
°C using the catalysts prepared by sol immobilisation method
Selectivity (%)
Catalyst Metal Au:Pd Conv Benzyl Benzal Benzoic Benzyl TON
(%) mol
ratio(%) alcohol dehyde acid benzoate
Au/C 1 1:0 0.2 9.0 81.9 0 8.1 11
Au-Pd/C 1 7:1 0.3 28.4 57.6 6.2 7.8 17
Au-Pd/C 1 3:1 1.5 1.8 63.4 3.1 31.4 95
Au-Pd/C 1 1:1.85 4.8 0.9 12.7 10.3 76.1 310
Au-Pd/C 1 1:2 5.3 1.2 8.3 11.1 79.3 350
Au-Pd/C 1 1:3 5.2 1.9 8.5 10.3 79.3 340
Au-Pd/C 1 1:7 4.3 9.6 13.6 7.3 69.5 280
Pd/C 1 0:1 1.6 3.9 56.4 3.3 36.4 105
Au/C +
Pd/C*1 1:1.85 1.6 0.8 26.2 11.4 61.6 105
Reaction conditions: T = 160 °C (433K), p O i = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 20 ml toluene, catalyst - variable, substrate/metal molar ratio = 6500, *Reaction of a physical mixture composed of 1 wt % Au/C and 1 wt % Pd/C. TON (moles of products/moles of metal) at 7 h reaction. TON numbers were calculated on the basis of total loading of metals.
3*
(Chapter 3 Qfbdpenti Tee (Eteidatitm o frCohiene
Table 3.6: Particle sizes o f metal clusters in mono and bimetallic supported catalysts
prepared by sol immobilisation
Catalysts* Au/C Pd/C Au-Pd/Cb Au/Ti02 Pd/T i02 Au-Pd/Ti02b
Mean
particle size
nm
4.7 6.3 3.7 4.6 4.8 3.9
Median
particle size
nm
a . i t j *
4.0 5.0 3.3
_ a a b A_J
3.9 3.8 3.5
a metal loading - 1 % total in all the catalysts, b Au.Pd = 1:1 wt (1:1.85 mol)
When Au-rich catalysts (1:0, 7:1, 3:1) were used, some benzyl alcohol was formed, but
this was readily oxidized to the main product benzaldehyde, as this sequential oxidation is
rapid at this temperature (5). We consider the initial oxidation of toluene to involve a surface
hydroperoxy intermediate formed from the interaction of the metal with oxygen and toluene.
However, as the fraction o f Pd in the alloy was increased (1:1.85, 1:2, 1:3, 1:7), the
selectivity to benzyl benzoate became dominant. The optimum catalyst composition found
was a 1:2 Au:Pd molar ratio (approximately 1:1 by weight or 1:1.85 by mol). The turnover
frequency (TOF: mole o f product formed per mole of metal per hour) o f toluene oxidation
after 7 hours o f reaction using this catalyst was ~50. For the sake of convenience, catalyst
having equal weight o f each metal (Au:Pd 1:1 wt or 1:1.85 mol) with lwt% total metal
loading has been used for further studies.
3S
&haptcr 3 QSkfhvnl-( ree (Oxidation o frCohtene
3.3. Attempts to increase the conversion of toluene
There were several approaches considered to increase the conversion of toluene under our
reaction conditions which are 160 °C temperature, 10 bar O2 partial pressure. The first
approach was to increase the catalyst amount in our reaction conditions as the number of
substrate molecules activated in a given time is proportional to the number of active sites. The
second approach was to increase the contact time (Reaction duration) between catalyst surface
and substrate molecules, So that the conversion of the substrate can be elevated. The
experimental results are shown in the following sections.
3.3.1. Effect of catalyst mass on toluene oxidation
4.5
coe©>coo
1000400 600 8002000
Mass of catalyst (mg)
Figure 3.3. Toluene conversion as a function of catalyst mass: T = 160 °C (433K), p O i = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml toluene, catalyst - 0.05-0.8 g 1 wt%AuPd/Csiw SI: sol-immobilisation method, w: weight ratio (1/1), substrate/metal mol ratio - 104000-6500, reaction time - 7 h. (Data table given in appendix, at the end of this chapter)
36
diopter 3 Q&ohvnt- rec (Oxidation ofTZokene
In this study, toluene oxidation reactions have been carried out using various masses of
catalyst. The catalyst employed in all these reactions was the most active l%AuPd/C (1:1 wt)
prepared by sol immobilisation. The other reaction conditions were remained unchanged. The
results reveal that increasing the catalyst mass has a linear effect on the conversion of toluene
(figure 3.3) and turnover frequency (TOF) remained constant indicating that there were no
diffusion limitations apparent.
2.5 n r 100
* 90
2.0 - ■ 80
- 70co£4>>Coo4)
- 60
- 50
- 40c4)3
• 30oi-0.5 - - 20
0.00 1 2 3 4 5 6 7 8
£
i t4)CO
Time (h)
Figure 3.4. Toluene conversion and selectivity to benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate: Reaction conditions: T = 160 °C (433K), / 7O2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml toluene, catalyst - 0.4 g lwt%AuPd/Csiw, SI: sol- immobilisation method, w: weight ratio (1/1). substrate/metal molar ratio = 13000, t = 7 h. Open symbol indicates conversion and solid symbols indicate selectivity. Key: ■ indicate selectivity to benzyl alcohol, ♦ indicate selectivity to benzaldehyde, A indicate selectivity to benzoic acid, • indicate selectivity to benzyl benzoate.
A typical reaction profile over an initial 7-hour reaction period using a higher substrate-
to-metal ratio at 160°C has been shown in figure 3.4. This experiment showed that the toluene
37
Chapter 3 Q^obetUi roo (Enndation o f <X5ohtene
conversion increases linearly over this time scale and the selectivity to benzyl benzoate also
increases as the selectivity to benzaldehyde decreases.
3.3.2. Effect of reaction tim e on toluene oxidation
In order to increase the conversion of toluene, the toluene oxidation experiment has been
prolonged for 48 h as the contact time (Reaction duration) between catalyst surface and
substrate molecules may play a significant role in increasing the conversion.
Table 3.7: Toluene oxidation performed for 48 hrs using the catalysts prepared by sol
immobilisation
Selectivity (% )
C atalystTim e
(h)
Conv
(% )
Benzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoateTON
l%Au-Pd/C 48 50.8 0.1 1.1 4.5 94.3 3300
1 % Au-Pd/Ti02 48 24.1 0.5 1.2 2.8 95.5 1570
Reaction conditions: T = 160 °c (43 :IK), pOi = 10 bar (0.1 MPa), stirring rate - 1500 rpm,substrate - 10 ml toluene, catalyst - 0.2 g, substrate/metal molar ratio = 6500, TON (moles of products/moles o f metal) at 48 h reaction. TON numbers were calculated on the basis of total loading of metals.
In this regime, much higher toluene conversions were attained and no CO2 was observed.
Once again only four products were observed: benzyl alcohol, benzaldehyde, and benzoic
acid in trace amounts and benzyl benzoate as the dominant product (-95% ) for both catalysts.
The carbon supported catalyst was typically twice as active as Ti02 supported catalysts over
this longer time scale, exhibiting a TON of 3300 with -3150 mol of benzyl benzoate per
mole of metal produced over the reaction period (Table 3.7).
From these two approaches, it is obvious that increasing catalyst mass and prolonging the
reaction time can facilitate the conversion of toluene under our reaction conditions. Hence, the
38
Chapter 3 Qfbobent-i free (Oxidation o f r&ohiene
further experiments have been designed to have low substrate to metal molar ratio and longer
reaction times.
3.4. Toluene oxidation using a lower substrate/metal ratio at longer reaction times
Toluene oxidation experiments have been carried out using a lower substrate/metal molar
ratio for longer hours to obtain high conversion. Initially, the most active l%AuPd/C was
tested at 160 °C under 10 bar O2 pressure. Samples were collected periodically through our
in-built sampling system in the autoclave in order to know the course of the reaction. The
conversion continued to increase steadily, fully depleting the toluene after 110 hrs (figure
3.5.). The selectivity o f benzyl benzoate was -86% at 110 hrs. To claim the contribution of
synergistic effect arose from gold and palladium, catalysts containing individual metal was
tested for toluene oxidation under the same conditions. The monometallic Au and Pd
catalysts showed very low conversion (8%) over 110 hours (Table 3.8), confirming again that
there is a synergistic effect in bimetallic gold palladium sol catalysts for toluene oxidation
system.
39
Chapter 3 Q&dpenti ree Oxidation o frColuene
££O0)a>jgcoCo>cooa>ca>3O
100
90
80
70
60
50
40
30
20
10
00 20 40 60 80 100 120
Time (h)
Figure 3.5. Toluene conversion and selectivity to benzyl alcohol, benzaldehyde, benzoic acid and benzyl benzoate:: Reaction temperature at 160 °C (433K) and 0.1 MPa pOj, 20 ml toluene, 0.8 g o f catalyst, toluene/metal molar ratio=3250 and reaction time: 110 h. Circles, lwt% AuPd/Csim, SI: sol-immobilisation method, m: molar ratio (Au/Pd=l/1.85). Open symbol indicates conversion and solid symbols indicate selectivity. Key: ■ indicate selectivity to benzyl alcohol, ♦ indicate selectivity to benzaldehyde, A indicate selectivity to benzoic acid, • indicate selectivity to benzyl benzoate.
40
Chapter 3 Qfbehvnt-t frce (Oxidation ofrCohtene
Table 3.8: Toluene oxidation performed using a lower substrate/metal molar ratio by
employing the catalysts prepared by sol immobilisation
Selectivity (%)
CatalystTime
(h)
Conv
(%)
Benzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoateTON
l%Au/C 0.5 0.7 35.6 53.1 6.9 4.4
7 0.8 6.9 82.8 3.3 7.0
24 2.5 0.8 77.1 13.9 8.2
31 3.3 0.7 69.3 21.3 8.7
48 3.9 0.5 57.4 32.7 9.4
72 5.5 0.3 38.1 51.3 10.3
96 7.0 0.2 28.1 61.3 10.4
103 7.9 0.2 24.8 64.2 10.8
110 8.1 0.2 23.7 64.9 11.2 264
!%AuPd/C* 0.5 0.9 2.1 21.4 1.8 74.7
7 6.9 0.6 5.7 3.0 90.7
24 31.8 0.2 2.0 5.3 92.5
31 43.4 0.2 1.7 5.8 92.3
48 58.1 0.3 1.0 5.4 93.4
72 73.8 0.2 0.9 8.6 90.3
96 93.4 0.1 1.0 10.9 88.0
103 98.1 0.1 1.1 13.8 85.1
41
(Chapter 3 QSxtbertt- rcc (Oxidation of X ohierw
110 99.3 0.1 0.2 14.2 85.6 3226.9
l%Pd/C 0.5 0.4 2.7 62.2 1.8 33.3
7 1.3 2.4 53.5 6.4 37.7
24 2.8 1.7 30.8 14.4 53.1
31 3.1 1.4 27.4 15.4 55.8
48 3.9 1.0 22.5 16.9 59.6
72 5.7 1.0 16.9 15.1 67.0
96 6.7 0.7 13.7 17.5 68.1
103 7.8 0.3 12.0 18.5 69.2
110 8.0 0.2 10.7 19.1 70.0 262
Reaction conditions: T = 160 °C (433K), / 7O2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 20 ml toluene, catalyst - variable amount, substrate/metal molar ratio = 3250, TON (moles o f products/moles of metal) at 110 h reaction. TON numbers were calculated on the basis o f total loading of metals. $ - 1:1 wt Au:Pd
42
Chapter 3 Q&obenti ree Oxidation o f rCohtene
The toluene conversion was also found to increase linearly with catalyst mass (figure 3.3)
Hence, with appropriate tuning o f catalyst loading and reaction conditions, it is evident that
complete and selective conversion o f toluene to the desirable products can be achieved (Table
3.9, show results o f an initial optimization).
Table 3.9: Toluene oxidation perform ed using a very low substrate/metal m olar ratio
by employing the catalysts prepared by sol immobilisation
Selectivity (% )
Time
(h)
Conversion
(% )
Benzyl
alcoholBenzaldehyde Benzoic acid
Benzyl
benzoateTONb
0.5 1.8 2.1 11.6 0.8 85.5 29
7 21.5 0.3 1.8 3.1 94.8 350
24 82.9 0.1 0.8 7.3 91.8 1347
27 94.4 0.2 1.0 13.3 85.5 1534
Reaction conditions: T = 160 °C (433K), /7O2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 20 ml toluene, catalyst - 1.6 g of l%AuPd/Csiw. SI - Sol immobilisation, w- 1:1 wt Au:Pd, substrate/metal molar ratio = 1625, TON (moles of products/moles of metal) numbers were calculated on the basis of total loading of metals.
Furthermore, toluene oxidation experiment has been done at the temperature little less
than 160 °C, which is 140 °C. The idea behind this experiment was to increase the selectivity
of benzyl benzoate to certain extent as the temperature can play a role of tuning the
selectivity distribution o f the products. The experiment had shown 95% yield of benzyl
benzoate with full conversion o f toluene (Table 3.10). The formation of benzyl benzoate is
discussed in chapter 4.
43
(?hapter 3 Qbdbm t free Oxidation o f Toluene
Table 3.10: Toluene oxidation performed at 140 °C by employing the catalysts prepared
by sol immobilisation
Selectivity (%)
Time
(b)
Conversion
(%)
Benzyl
alcoholBenzaldehyde Benzoic acid
Benzyl
benzoateTON
0.5 0.02 0 59.3 0 40.7
7 3.2 0.2 7.6 1.0 91.2
24 14.3 0.2 2.7 2.2 94.9
31 17.3 0.1 2.5 2.5 94.9
96 65.6 0.1 0.6 2.2 97.1
120 78.7 0.1 0.5 1.2 98.2
134 82.1 0.1 0.4 1.2 98.2
162 89.9 0.1 0.4 1.4 98.1
182 92.8 0.1 0.6 2.7 96.6
205 95.9 0.1 0.6 4.3 95.0
231 97.1 0.1 0.9 6.0 93.0 1578
Reaction conditions: T = 140 °C (433K), / 7O2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 20 ml toluene, catalyst - 0.8 g of l%AuPd/Csiw. SI - Sol immobilisation, w- 1:1 wt Au:Pd, substrate/metal molar ratio = 3250, TON (moles of products/moles of metal) numbers were calculated on the basis of total loading of metals.
(Phaptar 3 Qfbobent-t free (Oxidation op'Cohiene
3.5. Structure-activity relationship of Au-Pd bimetallic sol catalyst system
To determine the origin o f the differences in activity (Table 3.7) between the two
catalysts (based on carbon and TiC>2), we have characterized the common starting sol and the
two sol-immobilized m aterials using electron microscopy. Figure 3.6 shows a representative
scanning TEM (STEM ) high-angle annular dark field (HAADF) image o f the starting
colloidal particles deposited onto a carbon film.
Figure 3.6. A representative STEM HAADF image o f the starting Au Pd (1:1 wt) sol deposited onto a continuous carbon TEM grid. (Image taken by Ramachandra et al)
The bimetallic nanoparticles have a mean particle size o f 2.9 nm (figure. 3.7A) and were
found to be hom ogeneous Au-Pd alloys. More than 80% o f the particles were icosahedral or
decahedral (multiply tw inned), with the remainder being cub-octahedral or just single/double
twinned in character. They were also distinctly rounded in shape because they were still
coated with protective polyvinyl alcohol ligands.
6 /iaptor 3 Qbobenti jree Oxidation o frCohiene
Au+PdSol
B
Mean: 2.9 nmMedian: 3.2 nm
0 1 2 3 4 5 6 7 8 9 10 11 12 13Particle Size (nm)
(BSSfflAuPd/TiO^
Mean: 3.9 nmMedian: 3.5 nm
u 10
1 2 3 4 5 6 7 8 9 10 11 12 13Particle Size (nm)
35
30
^ 2 5
> J0u
I 15S'10
■s
i s■■.V
'
.
1 H A uPd/C
Mean: 3.7 nm Median: 3.3 nm
r~L3 4 5 6 7 8 9 10Particle Size (nm)
Figure 3.7. Particle size distributions for (A) starting Au-Pd colloidal sol, (B) so l- immobilized Au-Pd/TiC>2 dried at 120°C, and (C) sol-im m obilized Au-Pd/C dried at 120°C. (Image taken by Ram achandra et al)
46
Q&ohvntx^rve Oxidation o frColueneChapter 3
Figure 3.8. (A to F): Representative STEM HAADF micrographs o f Au-Pd nanoparticles in the Au-Pd/Ti02 sol-immobilized sample.
(G to L): Representative STEM HAADF micrographs o f Au-Pd nanoparticles in the Au-Pd/C sol-immobilized sample. (Image taken by Ramachandra et al)
*7
(Phapter 3 Qfbobenti free (Oxidation o frColiiene
■ M
Figure 3.9. A representative HAADF-STEM image o f the AuPd/Ti02 sol-immobilized catalyst after drying at 120 °C, in which it is clear that flat interfaces have developed between the metal particle and the support. (Image taken by Ramachandra et al)
After deposition o f the colloids onto either activated amorphous carbon or TiCh, the sol-
immobilized material was dried at 120 °C for 16 hours. The low-temperature drying process
in each case caused a very modest size increase in the Au-Pd particles, leading to HAADF-
measured mean sizes for the TiC>2- and carbon-supported catalysts o f 3.9 and 3.7 nm,
respectively (figure. 3.7 B and C).
The most important difference noted between the two samples relates to the morphology
o f the Au-Pd particles. In contrast to the starting colloid, many o f the smaller particles were
now found to be highly faceted, and primarily cub-octahedral (figure 3.8A to D) or singly
twinned (figure 3.8E) in character. Such particles preferentially exposed distinct {111} - and
(200}-type facets. The larger particles (figure 3.8F) still tended to be multiply twinned and
exclusively exposed {11 l}-type facet planes. In addition, the Au-Pd particles tended to form
an extended flat interface structure with the crystalline TiC>2 substrate (figure 3.9), which
could serve to improve particle adhesion and inhibit sintering at higher temperatures. In
48
Chapter 3 Qfbofoent- ree (Etodation o frCohiene
contrast, the corresponding HAADF images (figure 3.8 G to I) from the carbon-supported
samples show that the Au-Pd particles are more rounded and have a much lower ability to wet
the amorphous carbon support. The distribution of Au-Pd particle morphologies on the carbon
support was much closer to that o f the starting colloid, with icosahedral and decahedral (five
fold- twinned) particles predominating (figure 3.8 G to J) and comparatively few
single/double twinned (figure 3.8K) and cubo-octahedral (figure 3.8L) particles present. It is
plausible that the strong interaction of the Au-Pd metal with the TiC>2 support in the former
case templates many of the smaller multiply twinned particles to restructure as cub-octahedral
particles or simple single/ double-twinned crystals during the drying step.
X-ray photoelectron spectroscopy (XPS) analysis of the Au-Pd catalysts immobilized on
carbon and TiC>2 showed them to have a Pd/Au ratio o f 2.1 and 2.2, respectively, and
confirmed that in both cases, the Pd was predominantly in the metallic state. The AuPd/C
sample had approximately double the catalytic activity o f the AuPd/Ti02 sample, despite
having a very similar size distribution of Au-Pd particles. This suggests that simple metal
surface area considerations are not dominating the catalytic activity in this instance, because
the total numbers o f exposed surface atoms are almost identical (8), and the TONs per
surface-exposed atoms (Calculation of the number of surface exposed atoms is demonstrated
separately in 3.5.1) for the most-active catalysts are 1.03 * 104 for Au-Pd/C and 0.56 * 104 for
Au-Pd/TiC>2 (Table 3.7) (Tables 3.11 & 3.12). In addition, the similarity in surface
composition, as evidenced by XPS, does not provide a clue as to the source of the activity
difference.
The Au-Pd/ TiC>2 catalyst does clearly have more support/ particle periphery sites relative
to the Au-Pd/C catalyst, by virtue of its flatter, better wetting interface. However, such
periphery sites are probably not implicated in the catalytic process in this instance, because
the Au-Pd/Ti02 catalyst displays the lower activity. It is possible that the disparity in catalytic
49
(Chapter 3 Qfbofoent free (Oxidation o f rCohiene
activity may be related to subtle differences in the number o f low coordination facet-edge and
comer sites in the two cases. The flatter, more faceted, Au-Pd particles have fewer of these
low coordination number sites, whereas the more irregular, rougher Au-Pd particles have
substantially more o f them. If these low coordination number comer and/or edge positions are
implicated as active sites for toluene oxidation, then their relatively higher occurrence in the
Au-Pd/C sample could account for the superior performance displayed by this catalyst.
Another possible explanation could lie in the difference in the distribution of Au-Pd
particle morphologies found in the two catalyst samples. The Au-Pd/C catalyst predominantly
has multiply twinned (icosahedral and decahedral) particles, which tend to have {111} facet
terminations. In comparison, the Au-Pd/TiC>2 materials show an increased fraction of
cuboctahedral and singly/doubly twinned particles, which exhibit mixed {100}/ {111} facet
terminations. Hence, the increasing proportion of {100}-type facets in the Au-Pd/TiC>2 sample
correlates with a lowering o f the catalytic activity, and preparation strategies need to avoid
them.
SO
Chapter 3 Ofbobent-t rec (Oxidation o f Toluene
3.5.1. Calculation of the number of surface exposed atoms
Calculations o f the number o f exposed surface atoms were performed by Ramachandra et
al assuming that all the nanoparticles had either perfect icosahedral or cub-octahedral
morphologies. The number of atoms in the cluster as well as the number of exposed surface
atoms for a given cluster size is the same for both morphologies (9, 10). According to the
Mackay model an assembly o f n icosahedral shells about a central atom, contains N total
atoms where:-
N = (10/3)n} + 5n2 + (U /3)n + 1
The number o f atoms in the nlh shell is given by N„= 10n2 + 2
The total number o f surface atoms on activated carbon and Ti(>2 can then be calculated
as follows: Since the catalyst has a total metal loading o f 1 wt% on both supports, the number
o f Au and Pd atoms in 100 grams of the sample is 4.36 x 1021 and the number of exposed
surface atoms, assuming an equal distribution of gold and palladium on the surface, is then
2i 211.40 x 10 for the activated carbon support and 1.22x10 for the TiC>2 support.
The calculations for Au-Pd nanoparticles immobilized on activated carbon and TiC>2
supports are shown in Tables 3.11 and 3.12 respectively.
5 /
(Fhaptar 3 Q£x>betit free (Bfxidation of'toluene
Table 3.11. Calculation o f the percentage of exposed surface atoms based on the
measured particle size distribution for the AuPd/ activated carbon sample.
Cluster Size (nm)1
Frequency(% )b
Number of atoms in cluster'
Fraction of atoms on the surface (%)d
Percentage of total atoms'
Percentage of surface
atomsf
1.4 1.34 55 76.3 0.07 0.05
2.0 11.12 147 62.5 1.36 0.85
2.5 21.78 309 52.4 5.6 2.94
3.1 22.67 561 44.9 10.58 4.75
3.7 12.45 923 39.2 9.56 3.75
4.3 13.34 1415 34.7 15.7 5.45
4.8 4.45 2057 31.2 7.61 2.38
5.4 5.34 2869 28.3 12.73 3.61
6.0 2.67 3871 25.8 8.59 2.22
6.6 2.23 5083 23.8 9.4 2.24
7.2 1.34 6525 22.1 7.24 1.6
7.7 0.89 8216 20.5 6.08 1.25
8 3 0 10178 19.2 0 0
8.9 0 12431 18.1 0 0
9.5 0.45 14993 17.1 5.55 0.95
a Cluster size for n shell is calculated as to (2n+l)datom for n> l, where d at0m ~ 0.288 nm
b Frequency using the particle size distribution histogram
c,d Calculated from Mackay model
c,f Distribution of total atoms per cluster size calculated using the particle size distribution histogram
(a, b. c. d, e, f are co m m o n fo r T ab le 11 & 12)
(The above data supplied by Ramachandra et al)
S2
Chapter 3 Q&efoenl free (Oxidation 0 frCo!nem
Table 3.12. Calculation of the percentage of exposed surface atoms based on the
measured particle size distribution for the AuPd/ TiC>2 sample.
Cluster Size (nm)1
Frequency(%)b
Number of atoms in cluster0
Fraction of atoms on the surface (%)d
Percentage of total atoms0
Percentage of surface
atomsf
1.4 1.25 55 76.3 0.05 0.04
2.0 10.7 147 62.5 0.95 0.59
2.5 17.17 309 52.4 3.18 1.67
3.1 21.15 561 44.9 7.1 3.19
3.7 17.17 923 39.2 9.49 3.72
4.3 9.46 1415 34.7 8.01 2.78
4.8 5.73 2057 31.2 7.05 2.2
5.4 5.48 2869 28.3 9.4 2.66
6.0 3.74 3871 25.8 8.65 2.24
6.6 3.24 5083 23.8 9.84 2.35
7.2 1 6525 22.1 3.89 0.86
7.7 1.25 8216 20.5 6.12 1.26
8.3 0.75 10178 19.2 4.55 0.88
8.9 0.75 12431 18.1 5.56 1.01
9.5 0.5 14993 17.1 4.47 0.77
10.0 0 17885 16.1 0 0
10.6 0.25 21127 15.3 3.15 0.49
11.2 0.25 24739 14.6 3.69 0.54
11.8 0 28741 13.9 0 0
12.3 0.25 33153 13.3 4.94 0.66
(The above data supplied by Ramachandra et al)
S3
Chapter 3 Qfoebentx^free (Oxidation o frCohiene
3.6. Effect of temperature and pressure on toluene oxidation
Experiments have been designed with various temperatures and pressures in order to
know the influence of these experimental parameters on the rate of the reaction. The results
are shown in Table 3.13.
Table 3.13: Effect of temperature and pressure studied on toluene oxidation using sol
immobilised catalysts
Selectivity (%)
Catalyst* T°CpO i
bar
Time
h
Conv
%
Benzyl
alcohol
Benzal
dehyde
Benzoic
acid
Benzyl
benzoateTON
AuPd/C 160 10 48 50.8 0.1 1.1 4.5 94.3 3300
AuPd
/TiC>2160 10 48 24.1 0.5 1.2 2.8 95.5 1570
AuPd/C 120 10 48 10.6 0.2 7.1 13.1 79.7 690
AuPd
/TiC>2120 10 48 4.0 1.1 6.0 4.8 88.1 260
AuPd/C 80 10 48 0.9 8.6 34.2 0.1 57.2 60
AuPd/C 120 6 48 4.9 0.1 5.7 2.9 91.4 320
Reaction conditions: substrate - 10 ml toluene, catalyst - 0.2 g, substrate/metal molar ratio = 6500, stirring rate - 1500 rpm, TON (moles of products/moles o f metal) at 48 h reaction. TON numbers were calculated on the basis o f total loading o f metals. * - lw t % total metal loading (1:1 wt Au:Pd) for all the catalysts
The reaction performed at 120 °C, 10 bar O2 pressure has shown less conversion than the
reaction carried out at 160 °C, 10 bar O2 pressure by a factor o f 5 to 6., whereas the reaction
performed at 80 °C at 10 bar O2 pressure resulted in very low conversion. The reaction carried
out at 120 °C, 6 bar O2 pressure has shown 10 times less conversion than the reaction done at
160 °C, 10 bar O2 pressure.
S4
&hapia 3 Qfoohwtt- sfree (Oxidation o f rColuene
The rate o f any chemical reaction can be influenced by several factors. In general, a
factor that increases the number o f collisions between particles will increase the reaction rate
and a factor that decreases the number o f collisions between particles will decrease the
chemical reaction rate. Usually, an increase in temperature or pressure is accompanied by an
increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so
higher temperature implies higher average kinetic energy of molecules and more collisions
per unit time. Hence, the conversion o f toluene is directly proportional to the reaction
temperature and pressure too.
SS
Chapter 3 Qfbokvnt free (Oxidation o f rCokieno
References:
1. X. Li, J. Xu, L. Zhou, F. Wang, J. Gao, C. Chen, J. Ning, H. Ma, Catal. Lett. 110, 149 (2006)
2. F. Wang, J. Xu, X. Li, J. Gao, L. Zhou, R. Ohnishi, Adv. Synth. Catal. 347, 1987 (2005).
3. J. Gao, X. Tong, X. Li, H. Miao, J. Xu, J. Chem. Technol. Biotechnol. 82, 620 (2007).
4. N. Dimitratos et al., Phys. Chem. Chem. Phys. 11, 5142 (2009).
5. D. I. Enache et al., Science, 311, 362 (2006).
6. J. K. Edwards et al., Science, 323, 1037 (2009).
7. J.K. Edwards, B.E. Solsona, P. Landon, A.F. Carley, A. Herzing, C.J. Kiely and G.J. Hutchings, J. Catal. 236, 69 (2005).
8. A. Borodzinski, M. Bonarowska, Langmuir, 13, 5613 (1997).
9. A. Mackay, Acta Crystallographica, 15, 916 (1962).
10. J. M. Montejano-Carrizales, F. Aguilera-Granja, J. L. Moran-Lopez, Nanostructured Materials, 8, 269 (1997).
56
Chapter 3 Q^oberil free Oxidation o f rCohw?te
Appendix
Reproducibility of experiments:
In a final set o f experiments, the reproducibility of catalyst preparation method and
catalyst testing has been evaluated together (Table 3.14). Three trials have been done to study
the reproducibility. Two of them have been performed by one experimenter (Lokesh, trial 2
& 3) and one of them has been performed by another experimenter (Izham, trial 3). This way
it was possible to check the variations between the users. The results were in good agreement
with each other except the selectivity of benzoic acid and benzyl benzoate. This might be due
to the interruption o f oxygen flow occurred for a while at the end o f reaction, performed by
Izham. It has been claimed that oxygen concentration may affect the selectivity of ester as the
oxygen is involved in the transformation of hemiacetal (coupling o f aldehyde and alcohol) to
ester as shown in scheme 1 o f chapter 4. At the same time, low concentration of oxygen has
facilitated the over oxidation of benzaldehyde to benzoic acid.
Table 3.14: Liquid phase oxidation of toluene using three different batches of the same
catalyst
TrialConversion
%
Selectivity %
TOFBenzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoate
1 33.9 0.8 4.1 33.0 62.2 45.9
2 33.1 0.4 3.3 14.2 82.1 44.8
3 35.3 0.2 2.0 8.5 89.2 47.9
Reaction conditions: T = 60 °C (433 K),/?02= 10 bar (0.1 MPa), substrate - 10 ml tolu<catalyst - 0.2 g l%AuPd/Csi (1:1 wt) SI- Sol immobilisation, substrate/metal molar ratio = 6500, stirring rate - 1500 rpm, t = 48 hrs. TON (moles o f products/moles of metal) numbers were calculated on the basis o f total loading o f metals.
57
Chapter 3 Qfbebenti Tee (Oxidation o frCohwn6
Conversion profile with error bars5
4.5 4
3.5 3
2.5 2
1.5 1
0.50
0 1 2 3 4 5 6 87Time h
Figure 3.10a
Selectivity profile with error bars90
8070
□ Benzyl alcohol60
50 Benzaldehyde40
Benzoic acid30
Benzylbenzoate
20
Time h
Figure 3.10b
Figure 3.10 a&b: Toluene conversion & selectivity profile with error bars. aReaction conditions: Toluene - 188 mmol (20 ml), T = 160 °C, pC>2 = 10 bar, stirring rate 1500 rpm., Time - 7 h.
The Figure 3.10 a&b shows the variation in the catalytic data through error bars. The
error bars have been drawn based on the catalytic data from three different reaction runs by
using same batch of the catalyst. The median value of percentage conversion has been
considered to draw a reference graph. Further, positive and negative error values were added
58
Chapter 3 Q&okvnt-t freo (Oxidation of oliiette
to the reference graph by subtracting the numerical values (Conversion % & selectivity %)
from high to low on either side o f the median. The error bars shows that there is no much
difference in conversion as well as selectivity, stating that the selectivity here does not vary
as much as seen in Table 3.14. It suggests that the variation between catalyst batches is
greater than within the same batch. Also this observation shows much better agreement that is
in line with trial 2 & 3 (done by Lokesh), but not with trial 1 (done by Izham) in table 3.14.
The reason for the deviation in trial 1 has been already discussed. To conclude that the
toluene oxidation experiments are reproducible, this is evident from the Table 3.14 and figure
3.10 a&b.
S9
Chapter 3 Q&obetit frce (Stxidation o f rColuene
Table 3.15: Effect of catalyst mass studied on toluene oxidation
CatalystTime
h
Conv
%
Selectivity %
S/M TONBenzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoate
50 mg 1.0 0.093 9.3 55.4 27.5 7.7 104000 97.2
2.0 0.120 9.6 74.7 3.2 12.5 124.7
5.0 0.249 6 69.4 8 16.5 259
7.0 0.415 4.2 66 8.8 21 431
100 mg 1.0 0.185 6.6 69.7 8.6 13.5 52000 95.9
2.0 0.283 8.2 71.8 4.3 17.3 147.3
5.0 0.437 0.1 71.5 0.1 28.5 227
7.0 0.854 0.1 52.8 15.3 31.8 443.7
200 mg 1.0 0.316 5.3 76.2 2.4 16.1 26000 82.2
2.0 0.462 4 70.6 3.1 22.3 120.1
5.0 0.932 2.8 54.2 5.7 37.3 242.2
7.0 1.325 1.8 46.0 6.9 45.2 344.3
400 mg 1.0 0.419 3.6 47.7 4.2 44.4 13000 54.5
2.0 0.691 2.8 40.9 5.6 50.7 89.7
5.0 1.444 1.8 27.6 8.1 62.4 187.7
7.0 2.314 1.6 20.4 9.0 69.0 300.7
800 mg 1.0 0.947 2.0 35.6 6.1 56.3 6500 61.5
2.0 1.315 2.0 31.3 7.0 59.7 85.4
5.0 2.745 1.2 19.1 8.8 70.8 178.3
7.0 4.730 1.0 13.4 9.4 76.1 307.3
Reaction conditions: T = 160 °C (433K), /7O2 = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml toluene, catalyst - 1% AuPd/C (various amounts), S/M - substrate to metal molar ratio, TON - (moles o f products/moles of metal) at 7 h reaction. TON numbers were calculated on the basis o f total loading of metals.
60
Tolu
ene
Conv
ersi
on
%
(Phapter 3 Qfbobent- ^ree (Oxidation o frCokiette
Toluene Conversion profile for various amount of catalysts
4 -50 mg 100 mg 200 mg 400 mg 800 mg
1 2 3 4 5 6 7
Time h
Figure 3.11: Toluene conversion profile for various catalyst masses. aReaction conditions: Toluene - 376 mmol (40 ml), T = 160 °C, pC>2 = 10 bar, stirring rate 1500 rpm., Time - 7 h.
61
Chapter 4Toluene Oxidation and Catalyst <R$nse Studies
&hapter$ rCohierK (Oxidation and OPatatyst (s&ense Q&tndies
4.1. Outline
On continuing the studies o f toluene oxidation which were dealt with chapter 3,
this chapter discusses the possible routes for the formation of benzyl benzoate, which is
the major product under the current reaction conditions of liquid phase oxidation of
toluene using sol immobilised catalysts. Further, experimental results to demonstrate
the reusability of the catalysts with the evidence o f electron microscopic images have
been detailed. Finally, the preliminary results from the solvent free oxidation of
substituted toluenes using carbon supported Au-Pd bimetallic nanoparticles prepared by
sol immobilisation are shown.
4.2. Discussion about the formation of benzyl benzoate
Toluene is oxidized to benzyl alcohol, benzaldehyde and benzoic acid through the
reactive hydroperoxy species as Colby et al (1) and W. Partenheimer (2) demonstrated.
These transformations are shown in step 1, 2, 3, and 4. It is considered that the high
selectivity to benzyl benzoate could result from four possible mechanisms of either
coupling o f the benzaldehyde with the benzyl alcohol to give the hemiacetal, followed
by oxidation to the ester as shown in scheme 1 or direct thermal (catalyzed) dehydrative
esterification between benzoic acid with alcohol scheme 2 or Cannizzaro reaction
between benzyl alcohol with benzaldehyde via alkoxide as shown in scheme 3 or
Tishchenko coupling o f two benzaldehyde molecules via “dimer” directly to the ester as
shown in scheme 4.
Recently, BSumer and co-workers (3) have shown that methyl formate is formed on
an Au catalyst when methanol is oxidized with O2 under very dilute conditions; we
consider that this precedent favours mechanism of coupling of aldehyde and alcohol in
62
(Chapter 4 rColuene ®scidatwn and ataipst &tease Q&tudies
the present case as shown in scheme 1. We performed several control experiments to
rule out the other three possible pathways. Heating the oxidized intermediates with O2 in
the absence o f catalyst gave little ester (Table 4.1), establishing the involvement of the
catalyst in steps beyond the initial toluene-to-alcohol oxidation.
Step: 1
2 II J + 2 0 2
Toluene
Step: 2
*CH2
+ »OH
Step: 3
G 0HBenzyl alcohol
2 + 2 *0-0H
1
0 2 + 2 *OH
fT^ O H
Benzyl alcohol
a1
^ ° - + H-
T
a + H*
Benzaldehyde
Step:4
Benzaldehyde
*OH
OH+ H*
Benzoic acid
63
Chapter 4 rCetuene ( delation and (Pataipst (Reuse Q&tudies
Heating mixtures o f benzyl alcohol with benzoic acid in the absence of O2, with and
without catalyst, did not result in ester (Benzyl benzoate) formation (Tables 4.3 & 4.2),
ruling out direct esterification (Scheme 2). Similarly, treating alcohol with
benzaldehyde also failed to produce ester (Tables 4.2 & 4.3), ruling out the Cannizzaro
mechanism (scheme 3). Finally, we investigated reactions of the benzaldehyde alone in
the absence of O2, with and without catalyst, and again observed no ester formation
(Tables 4.3 & 4.2), precluding the Tishchenko mechanism (scheme 4). Hence we
conclude that the high selectivities observed are consistent with the involvement of the
hemiacetal (scheme 1).
Scheme 1:
► Ph
O
Hemiacetal
Ph + 2H*
Benzyl benzoate
Scheme 2:O
Benzoic acid Benzyl alcohol Benzyl benzoate
64
(Chapter % rCohtone (Oxidation and (?ata1pst (s&ense Q&tudies
Scheme 3:
Cannizaro reaction
Benzyl alcohol
OH
*C
Alkoxide
O
Benzyl benzoateHeat
Benzoic acid + Benzyl alcohol
Scheme 4:Tischenko reaction
C °II2 Ph - C — H
?CH2Ph
wO
Benzyl benzoate
PhCH^a
Chapter $ rCohiene Oxidation and &aia >st (s&euse Qfotndies
Table 4.1. Liquid phase reaction of benzyl alcohol, benzaldehyde and benzoic acid
in toluene at 160 °C without catalysts in the presence of O2.
Moles
ReactionTime
(h)Toluene Benzyl
alcohol Benzaldehyde Benzoicacid
Benzylbenzoate
0.3M (PhCH2OH + PhCHO) in toluene 0 0.36430 0.00600 0.00600 0.00000 0.00000
0.5 0.36550 0.00451 0.00580 0.00049 0.00000
1 0.36520 0.00443 0.00585 0.00078 0.00002
2 0.36510 0.00445 0.00598 0.00071 0.00004
5 0.36500 0.00421 0.00605 0.00091 0.00007
7 0.36500 0.00381 0.00611 0.00122 0.00009
0.3M (PhCH2OH + PhCOOH) in toluene 0 0.36430 0.006 0.00000 0.00600 0.00000
0.5 0.36880 0.00461 0.00011 0.00262 0.00010
1 0.36780 0.00448 0.00055 0.00331 0.00015
2 0.36690 0.00441 0.00101 0.00372 0.00022
5 0.36630 0.00425 0.00163 0.00386 0.00022
7 0.36620 0.00383 0.00224 0.00375 0.00020
0.3M PhCHO in toluene 0 0.36430 0.00000 0.01200 0.00000 0.00000
0.5 0.36468 0.00045 0.00851 0.00111 0.00155
1 0.36400 0.00061 0.00885 0.00171 0.00114
2 0.36362 0.00077 0.00829 0.00245 0.00117
5 0.36322 0.00109 0.00724 0.00349 0.00125
7 0.36276 0.00138 0.00683 0.00433 0.00100
Reaction conditions: Toluene = 40 ml, T = 160 °C, pO i = 10 bar, Stirring rate = 1500 rpm
66
(Chapter % rCohtene (Oxidation and 6?atatyst (Rouse Qfbtudies
Table 4.2. Liquid phase reaction of benzyl alcohol, benzaldehyde and benzoic acid in
toluene at 160 °C without catalysts in the presence of He.
Mol
ReactionTime
(h)Toluene Benzyl
alcohol Benzaldehyde Benzoicacid
Benzylbenzoate
0.3M (PhCH2OH + PhCHO) in toluene 0 0.36430 0.00600 0.00600 0.00000 0
0.5 0.36371 0.00622 0.00619 0.00018 0
1 0.36440 0.00582 0.00591 0.00018 0
2 0.36440 0.00580 0.00585 0.00025 0
5 0.36414 0.00573 0.00619 0.00024 0
7 0.36562 0.00517 0.00523 0.00028 0
0.3M (PhCH2OH + PhCOOH) in toluene 0 0.36430 0.00600 0.00000 0.00600 0.00000
0.5 0.36717 0.00600 0.00004 0.00310 0.00000
1 0.36663 0.00587 0.00006 0.00374 0.00001
2 0.36664 0.00583 0.00006 0.00375 0.00001
5 0.36663 0.00582 0.00010 0.00405 0.00003
7 0.36706 0.00547 0.00011 0.00361 0.00005
0.3M PhCHO in toluene 0 0.36430 0.00000 0.01200 0.00000 0
0.5 0.36383 0.00003 0.01190 0.00054 0
1 0.36351 0.00002 0.01215 0.00061 0
2 0.36415 0.00003 0.01146 0.00066 0
5 0.36481 0.00003 0.01086 0.00061 0
7 0.36618 0.00003 0.00992 0.00016 0
Reaction conditions: rpm.
oluene = 40 ml, T = 160 °C, pHe = 10 bar, Stirring rate = 1500
67
Phapter 4 IPohiene (Etiridatien and (Patatyst (Reuse Q&tudies
Table 4.3. Liquid phase reaction of benzyl alcohol, benzaldehyde and benzoic acid in
toluene at 160 °C with catalysts in the presence of He.
Mol
ReactionTime
(h)Toluene Benzyl
alcohol Benzaldehyde Benzoicacid
Benzylbenzoate
0.3M (PhCH2OH + PhCHO) in toluene 0 0.3643 0.0060 0.0060 0 0
0.5 0.3659 0.0045 0.0059 0 0
1 0.3660 0.0044 0.0059 0 0
2 0.3663 0.0042 0.0058 0 0
5 0.3664 0.0040 0.0059 0 0
7 0.3667 0.0037 0.0059 0 0
0.3M (PhCH2OH + PhCOOH) in toluene 0 0.3643 0.0060 0.0000 0.0060 0
0.5 0.3698 0.0037 0.0009 0.0018 0
1 0.3691 0.0037 0.0010 0.0025 0
2 0.3690 0.0036 0.0010 0.0027 0
5 0.3689 0.0034 0.0010 0.0029 0
7 0.3690 0.0033 0.0011 0.0029 0
0.3M PhCHO in toluene 0 0.3643 0.000000 0.012000 0 0
0.5 0.3643 0.000000 0.011980 0 0
1 0.3645 0.000000 0.011766 0 0
2 0.3646 0.000000 0.011690 0 0
5 0.3658 0.000000 0.010516 0 0
7 0.3651 0.000000 0.011170 0 0
Reaction conditions: Toluene = 40 ml, T = 160 °C, pC>2 = 10 bar, Stirring rate = 1500 rpm, 0.4 g o f catalyst, 1 wt% (Au-Pd)/Csiw, SI- sol immobilisation, w - 1:1 wt Au.Pd
68
(Chapter ■¥ rCeluene (S&idatien and 6*atatyst (Reuse Qfbtudies
4 .3 . Catalyst reusability studies:
In this study, stability o f the catalysts has been investigated for the liquid phase
oxidation o f toluene. Because, it is crucial to confirm that high-activity catalysts can be
reused. With the l%Au-Pd/Ti02 catalyst prepared by sol immobilisation the reaction
was stopped after 7 hours, and the catalyst was recovered by decantation. Then, the
recovered catalyst was loaded in the reactor containing fresh toluene and tested under
the same reaction conditions. Identical conversion was obtained on reuse of the 1 %Au-
Pd/TiC>2 catalyst (Table 4.4).
Table 4.4: Liquid phase oxidation of toluene to evaluate the used catalyst.
Selectivity (%)
Catalyst5Time
h
Conv
%
Benzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoateTON
aAu-Pd/Ti02 7 2.1 2.9 6.6 1.0 89.5 135
bAu-Pd/Ti02 7 2.2 2.2 6.5 2.3 89.0 141
Reaction conditions: T = 160 °C (433K), pC>2 = 10 bar (0.1 MPa), substrate - 20 m toluene, catalyst - 0.4 g, substrate/metal molar ratio = 6500, stirring rate - 1500 rpm, TON (moles o f products/moles of metal) at 7 h reaction. TON numbers were calculated on the basis of total loading of metals. 5 - lw t % total metal loading (1:1 wt Au:Pd) prepared by sol immobilisation.a - fresh catalyst, b - used catalyst.
For the l%Au-Pd/C catalyst prepared by sol immobilisation, the reaction was
stopped after 7 hours and the catalyst was allowed to settle by centrifugation. The liquid
phase was then carefully removed by decantation and fresh toluene was added to the
catalyst to perform the reaction. The reaction was then allowed to proceed for a further 7
hours, and the whole process was repeated a further three times. The reaction profile
obtained with the decantation experiments was identical to that obtained with the fresh
69
Chapter 4 (T5ohtene (Steidatien and d?ataipst (Reuse Q&tudies
catalyst (figure 4.1). The decanted liquid was analysed for leached metals using atomic
absorption spectroscopy and inductively coupled plasma spectroscopy and metal
concentrations were below the detection limits.
800 n
600 -
O 400 -
200 -
0 5 10 15 20 25 30
Time (h)
Figure 4.1. Standard reaction of AuPd/C compared with decantation of the reaction mixture at 7, 14, 21 and 28 h and fresh toluene added and the reaction continued. Reaction conditions: T = 160 °C (433K), /7O2 = 10 bar (0.1 MPa), substrate - 20 ml toluene, catalyst — 0.4 g l%AuPd/Csi (1:1 wt) SI- Sol immobilisation, substrate/metal molar ratio = 6500, stirring rate - 1500 rpm, t = 28 h. TON (moles o f products/moles of metal) numbers were calculated on the basis of total loading of metals. Key: □ indicates TON obtained with standard reaction, x indicates TON obtained with decantation procedure.
One of the key factors that must be considered for heterogeneous catalysts
operating in three-phase systems is the possibility that active components can leach into
the reaction mixture, thereby leading to catalyst deactivation or, in the worst case, to the
formation of an active homogeneous catalyst. No metal was observed to have leached
70
(Chapter % rCohiene (Oxidation and (TPatalpst 0ieuse Q&tndies
into the liquid phase under the present reaction conditions. It is confirmed by an
experiment, which involves the decantation of liquid phase after certain hours of
reaction to remove the catalyst, and then subjecting the decanted part for the oxidation
in the absence o f catalyst. The decantation should be done at relatively hot condition
(~50 °C), so that it is possible to find out whether the metals have got leached or not at
higher temperature. The decanted liquid showed no further reaction in the absence of
catalyst (figure 4.2 and table 4.5).
15
g 10Conk.•>C
0 1 1 1 1
0 2 4 6 8 10
Time (h)
Figure 4.2: Effect o f removing the catalyst by decantation at the reaction temperature.
Rem oval of Au-P d/C
71
Chapter % rCoktene (Oxidation and 6*atafyst Chaise Qfbtudies
Table 4.5: Investigation of leaching of metals from the catalyst during the liquid phase
oxidation o f toluene
Time ConvSelectivity (%)
TON(h) (%)
Benzyl
alcoholBenzaldehyde
Benzoic
acid
Benzyl
benzoate
0.5 1.3 3.3 12.7 5.3 78.7 21.5
2 6.8 0.9 5.1 2.4 91.6 110.4
4* 10.1 0.4 5.7 6.4 87.4 164.6
6 10.4 1.2 6.7 11.1 81.1 169.6
8 10.5 0.6 7.2 11.3 80.9 171.3
Reaction conditions: T = 160 °C (433K), / 7O2 = 10 bar (0.1 MPa), substrate - 20 ml toluene, catalyst - 0.8 g l%AuPd/Csi (1:1 wt) SI- Sol immobilisation, substrate/metal molar ratio = 3250, stirring rate - 1500 rpm, TON (moles o f products/moles of metal) numbers were calculated on the basis o f total loading of metals. # After 4h, the liquid was decanted at the reaction temperature to remove the catalyst and the decanted liquid was allowed to proceed for oxidation in the absence of catalyst. No reaction is observed. The decanted liquid was analysed for Au and Pd and levels were below detection levels.
Detailed STEM characterization shows that there is minimal particle growth or
morphology change for the Au-Pd/C catalysts when studied over an extended reaction
period (figure 3.5 - chapter 3), during which catalysts were recovered after 31 and 65
hours o f reaction, followed by two reuse cycles o f 7 hours (figures 4.3 and 4.4).
Therefore, it is clear that any sintering or structural modification o f these highly active
catalysts is minimal, and it is considered that the catalysts are stable and reusable.
72
Chapter 4
A
Mean: 3.7nm Median: 3.3 nm
20
1 2 3 4 5 6 7 8 9 10Particle Size (nm)
C
\ i AuPd/C after 65 h reaction
Mean 4.8 nm Median: 4.3 nm
2 4 6 8 10 12 14 16 18 20
Particle size (nm)
rCohtene (Oxidation and (Pataipst (Reuse (Studies
B
D
1 lAuPd/C after 79 h reaction30Mean: 4.5 nm Median: 4.0 nm
2 4 6 8 10 12 14I ■
16 18 20Particle Size (nm)
Figure 4.3. Particle size distributions for (A) unused sol imm obilized AuPd/C catalyst, (B) sol immobilized AuPd/C after 31 h reaction, (C) sol imm obilized AuPd/C after 65h reaction , (D) sol immobilized AuPd/C after 65 h reaction and two subsequent re-uses for 7 h giving a total time-on-line o f 79h. It is clear that re-use and multiple use cycles cause only a very m inor increase in median AuPd particle size. (The above data supplied by Ramachandra et al)
73
• l AuPd/C after 31 h reaction
Mean: 4.2 nm Median: 3.5 nm
14 16 18 20Particle size (nm)
&hapter 4 rCoktene (Oxidation and (Patalpst (spouse Qdbtndies
Figure 4.4: Typical STEM -HAADF images o f AuPd nanoparticles o f the re-used catalysts. (A, B, C) sol immobilized AuPd/C after 31 h reaction: (D, E, F) sol immobilized AuPd/C after 65 h reaction , and (G,H,I) sol immobilized AuPd/C after 65 h reaction and two subsequent re-uses for 7 h giving a total time-on-line o f 79 h. (The above data supplied by Ramachandra et al)
74
Chapter % 'Toluene (&adatwn and (?atalpst G&euse Q&tudies
4.4. Solvent free oxidation of substituted toluenes using carbon supported Au-Pd
bimetallic nanoparticles prepared by sol immobilisation
This part deals with the general applicability of the carbon supported Au-Pd
bimetallic alloy nanoparticles prepared by sol immobilisation in various heterogeneous
oxidation catalysis systems. This material has been reported as an active catalyst for
toluene oxidation under solvent free conditions in the previous two chapters. Having
relevance with toluene, substituted toluenes such as methoxy toluenes, nitro toluenes,
and xylenes are oxidized using this catalyst, again with no use of solvent. The oxidation
of substituted toluenes is another interesting area to explore since these molecules are
employed as feed stocks in the fine chemical industries. Often, these starting materials
are oxidized by toxic and expensive organic reagents to get high turnover of the
products. Hence, these processes are not environmentally benign and commercially
viable as they produce much waste. It is reported here that Au-Pd bimetallic alloy
nanoparticles prepared by sol immobilisation can be utilised to oxidise these feed stocks
in the absence o f any toxic oxidising reagents and solvents.
The liquid phase oxidation of 2-, 3-, and 4- methoxytoluenes (Table 4.6), and 2-, 3-,
and 4-nitrotoluenes (Table 4.8), have been studied under the same conditions of toluene
oxidation as it is reported in chapter 3.
7S
Chapter 4 rOahiene (£)sddation and ffataipst (Reuse Qfbtudies
Table 4.6. Comparison o f the catalytic activity of substituted toluenes in the
absence of solvent with O2.
Selectivity (%)
SubstrateTime
(h)
Conv
(%)
Benzylalcohol Benzaldehyde
Benzoic
acid
Benzylbenzoate TON
Toluene 1 1.1 2.7 27.5 6.9 62.9
4 2.2 2.2 18.7 8.6 70.5
7 4.4 1.0 12.4 10.4 76.2 284
n-Methoxy
benzylalcohol
n-Methoxybenzaldehyde
n-Methoxybenzoic
acid
n-Methoxy benzyl n- methoxy benzoate
Toluene Esters C-Cproducts
4-Methoxy
toluene
1 2.3 0.8 18.7 30.3 24.1 0.9 7.1 18.1
4 6.1 0.8 11.8 25.8 43.5 0.2 5.0 12.9
7 10.6 0.5 7.4 26.9 49.0 0.1 4.5 11.6 581
3-Methoxy
toluene
1 1.7 1.4 16.4 46.5 8.1 7.4 1.2 19.0
4 2.0 1.1 33.3 30.6 2.2 19.5 0.8 12.5
7 3.7 0.9 12.6 43.3 17.0 6.8 1.2 19.0 204
2-Methoxy
toluene
1 3.6 1.4 8.8 63.8 0.7 22.7 0.3 2.3
4 6.4 1.0 10.0 66.8 3.6 15.7 0.4 2.5
7 11.1 0.8 13.0 61.1 7.3 11.8 0.8 5.2 619
76
Chapter 4 T ohieno (Oxidation and (?ataipst (sReuse Qfbtudies
Reaction conditions: T = 160 °C (433K), pC>2 = 10 bar (0.1 MPa), substrate - 20 ml, catalyst - 0.4 g l%Au-Pd/C (1:1.85 Au:Pd mol ratio) prepared by sol-immobilisation method, stirring rate - 1500 rpm, TON (moles of products/moles of metal) at 7 h reaction. TON numbers were calculated on the basis of total loading of metals. $ - lwt % total metal loading (1:1 wt Au:Pd) prepared by sol immobilisation.
Table 4.7. Additional oxidation products for methoxy toluene derivatives.
Methoxy
toluene Additional Oxidation Product Structures
Derivatives
R = 2- OMe,
3- OMe,
4- OMe
In all the reactions, oxidised products containing the following functional groups; -
OH, -CHO, -COOH, -COOR’ and other by products have been identified. No CO2 was
observed in all the reactions, and the reactivity trend (4-methoxy— 2-methoxy- >3-
methoxy ~ toluene > 2-nitro- > 3-nitro ~ 4-nitro) is indicative of the involvement of
77
Chapter 4 rCohtme <&ridati<jn and €?atalpst 0teuse Q&tudies
electron-deficient intermediate(s). The difference in activity could be attributed by the
contribution o f mesomeric effect coupled with hyper-conjugation, which is common in
aromatic compounds having electron donating group (-OCH3) and electron withdrawing
group (-NO2) as demonstrated by M. Szwarc (2).
Table 4.8. Liquid phase oxidation of 2-, 3-, 4- nitrotoluenes at 160 °C
Selectivity (%)
Substrate Time
(h)
Conv
(%)
rt-Nitro
benzyl
alcohol
w-Nitro
benz
aldehyde
/t-Nitro
benzoic
acid
Benzaldehyde TolueneBenzyl
benzoate
TON
2- Nitro
toluene1 1.1 2.9 1.0 7.3 6.0 82.8 0.0
4 1.5 4.3 1.4 8.8 7.1 76.4 2.0
7 2.4 5.7 1.7 25.0 6.8 57.2 3.6 142
3-Nitro
toluene1 0.5 6.8 3.8 0.0 5.6 83.6 0.2
4 0.8 6.1 2.2 5.3 9.2 75.6 1.6
7 1.1 9.5 4.9 9.3 8.2 62.3 5.8 66
4-Nitro
toluene1 0.2 0.1 13.5 0.0 0.0 86.4 0.0
4 1.0 2.7 10.9 0.0 9.1 77.3 0.0 68
Reaction conditions: T = 160 °C (433K), pO i = 10 bar (0.1 MPa), substrate - 20 ml, catalyst - 0.4 g l%Au-Pd/C (1:1.85 Au:Pd mol ratio) prepared by sol-immobilisation method, stirring rate - 1500 rpm, TON (moles o f products/moles o f metal) at 7 h reaction. TON numbers were calculated on the basis of total loading of metals.
These effects ultimately affect the bond energy of the methyl group in
substituted toluenes which causes the difference in activity (2). There are some
78
Chapter 4 rCohiene (Oxidation and ( j’atalpst GReuse Q&tudies
additional products were formed during the oxidation of methoxy toluenes, which are
identified as a family o f esters and C-C coupling products (Table 4.7).
The liquid phase oxidation o-,m-,p- xylenes (Table 4.9) have been also investigated at
160 °C. The catalysts are equally effective for the oxidation of these substrates and
formed the aldehyde, acid, and esters as products, with the relative amounts being
dependent on the conversion (Table 4.9), confirming the wider applicability of the
catalyst, l%AuPd/C (1 :lwt). Again the bond energy difference due to the association of
mesomeric effect and hyper-conjugation makes these molecules to oxidize at different
rates as demonstrated by M. Szwarc.
79
Chapter 4 rCohiene (Jactation and (?atatyst C&ense Qfbtudies
Table 4.9. L iquid phase oxidation of o-,m-,p- xylenes a t 160 °C
Selectivity (%)
SubstrateTime
(h)
Conv.
(%)
w-Toluyl
alcohol
n-
Tolualdehyde
w-Toluic
Acid
C-C
products/EstersTONd
o-xylene8 1 0.2 2.2 50.2 7.6 40.0
4 0.5 0.7 29.1 15.0 55.2
7 0.7 1.8 21.6 18.6 58.0 42b
m-xylenea 1 0.1 0.0 65.8 0.0 34.2
4 0.3 0.0 38.7 3.7 57.6
7 0.5 1.2 27.0 12.4 59.4 25"
/>-xylenea 1 0.1 0.0 49.5 10.5 40.0
4 0.4 1.3 29.0 19.0 50.7
7 0.6 0.0 20.9 26.6 52.5 34b
p-xyleneb 24 2.8 1.3 5.7 18.5 74.5
31 3.6 1.5 5.7 20.5 72.3
48 6.8 1.3 4.6 26.2 67.9 171c
Reaction conditions: T = 160 °C (433K), p02 = 10 bar (0.1 MPa), substrate - 20 ml, catalyst -l% A u-Pd/C (1:1.85 Au:Pd mol ratio) prepared by sol-immobilisation method, substrate/metal molar ratio =5600a (0.4 g catalyst), substrate/metal molar ratio = 2800b (0.8 g catalyst) stirring rate - 1500 rpm, TON (moles o f products/moles of metal) numbers were calculated on the basis of total loading of metals.
80
Chapter 4 rCohie}ie (Qadatwn and (Patatyst GPense Qfbtudies
References:
1. J. Colby, D. I. Stirling, H. Dalton, Biochem. J. 165, 395 (1977).
2. W. Partenheimer, Catal. Today 23, 69 (1995).
3. A. Wittstock, V. Zielasek, J. Biener, C. M. Friend, M. Baumer, Science, 327, 19
(2010).
4. M. Szwarc , J. Chem. Phys. 16, 128 (1948)
81
Chapter 5SoCvent-(Free Oxidation of<BenzyCfiCcohoC
(Chapter S Q&obent- ree (Oxidation ofO&cnzpl alcohol
5.1. Outline
This chapter deals with the results obtained from the liquid phase oxidation of
benzyl alcohol using supported AuPd alloy nanoparticles as catalysts under solvent free
conditions.
Supported metal nanoparticles are receiving significant attention as catalysts for a
broad range of selective chemical transformations. In particular, Au nanoparticles have
been shown to be particularly effective for a range o f redox reactions (1-7). These
include the low temperature oxidation of CO (7) especially when used as part of the
processes for purifying a fuel cell feedstock (8) the synthesis o f vinyl chloride by the
hydrochlorination o f ethyne (9) the selective oxidation o f alkenes to epoxides (10, 11)
and alcohols to aldehydes (12) as well as selective hydrogenation (13). Recently, it is
reported that the alloying o f Pd with Au can enhance the activity o f the nanoparticles,
and such materials have been found to be particularly effective for the direct synthesis of
hydrogen peroxide from H2 and O2 (14, 15) and the oxidation of alcohols (16). As a
matter of resemblance with toluene and experimental proof for oxidation catalysis, the
supported Au-Pd bimetallic alloy nanoparticles prepared by sol immobilisation were also
tested for benzyl alcohol under solvent free conditions at mild temperature.
82
Chapter S Q&ohvnti free (Oxidation of O&onzpl alcohol
5.2. Solvent free oxidation of benzyl alcohol using supported Au-Pd bimetallic
nanoparticles prepared by sol immobilisation:
The synthesis o f a systematic set of Au-Pd colloidal nanoparticles having a range of
Au/Pd ratios has been examined for benzyl alcohol oxidation as it was demonstrated
already for toluene oxidation (chapter 3 - Table 3.5). The catalysts have been
structurally characterized using a combination o f UV-visible spectroscopy, transmission
electron microscopy, STEM HAADF/XEDS, and X-ray photoelectron spectroscopy.
The Au-Pd nanoparticles are found in the majority of cases to be homogeneous alloys,
although some variation is observed in the Au-Pd composition at high Pd/Au ratios. The
optimum performance for the oxidation of benzyl alcohol is observed for a catalyst
having a Au/Pd 1:2 molar ratio. These measured activity trends are discussed in terms of
the structure and composition of the supported Au-Pd nanoparticles.
5.2.1. Results and discussion
The Au, Pd, and Au-Pd colloidal sols (Au/Pd= 1:0, 7:1, 2:1, 1:1, 1:1.85, 1:2, 1:7, 0:1)
were prepared and examined by UV-visible spectroscopy. For the pure Au sol, the
appearance of the plasmon resonance was observed at 505 nm (figure 5.1). This
resonance peak is characteristic for gold nanoparticles with particle sizes below 10 nm
(17, 18). In the case o f the Pd monometallic sol, there is no surface plasmon band (17).
The spectra resulting from the AuxPdy bimetallic sols show the disappearance of the gold
surface plasmon band on addition of Pd, as previously reported for this system (17, 19)
which has been ascribed to changes in the band structure of the Au particles due to
alloying with Pd. These results suggest the formation of bimetallic random-alloy
nanoparticles, which is in agreement with previously reported data (20-22).
83
Chapter S Q&okvnt free (Oxidation of<S&enzpl alcohol
Au 7Au:1 Pd 3Au:1 Pd 2Au:1 Pd
- 1Au:1.85Pd 1Au:2Pd 1Au:3Pd
1Au:7Pd Pd
a>ac03€o(/>-Q<
300 400 500 600 700 800
Wavelength (nm)
Figure 5.1. UV-Vis spectra obtained from the various AuxPdy colloidal solutions.
Representative bright field TEM micrographs (figure 5.2 & 5.3) were acquired in
order to measure the particle size distributions o f the colloidal metal nanoparticles after
immobilization and drying on the activated carbon support. The particle size data for all
the colloid compositions used in this study are summarized in Table 5.1. The median
particles sizes for the monometallic Au and Pd colloids were found to be 3.5 and 5.4 nm,
respectively. The median particle sizes for all the AuxPdy alloys were in the 2.9- 4.6 nm
range, and did not show any discernible systematic size trend with colloid composition.
8*
Chapter S Q&vAvnt- yfrar (Oxidation o f O&onqd alcohol
• V V ?20 nm
* III ' llPfllllll(b) Au:Pd=7:l|fc > *
♦ VV T * *
v f l f c • % ’ %1 *•> jL* ^ . tV ^ v * * lgp.%
* *%4BEb ■«. « JiP-'.jr-y-* ■Jl* ■ v *# % . ■ . ■20’nm
_
(d) Au:Pd=l:l
% : * >* ** * n ‘r
\u:Pd=l:1.85
C ■ -! '
i •.(f) Au:Pd=l:2* m */• •V * * • ; v W * ; * > •'* *V
M • ■It ^ * *V/.* j | ( r %» " •
f - v *
Lfg) Au:Pd=l:7
S L # £ } * ? . \ • tH l V -# t - ,
y *’c * k ,*
i /E tF - .X * * _ •*^ *>» __1_
v * ? * • *
Figure 5.2: Representative bright field TEM micrographs showing AuPd particles immobilised on the activated carbon support: (a) Au only; (b) A u:Pd=7:l; (c) A u:Pd=2:l; (d) A u :P d = l:l; (e) Au:Pd=l:1.85; (0 A u:Pd= l:2 ; (g) A u:Pd=l:7; (h) Pd only. (Images taken by Ram et at)
8S
Chapter S QfbokvtU-< frce Oxidation of O&empt alcohol
40
35
30£
2S
c 20■3 15a
10u .
5
0
1 I ' iAu/C
— M ean: 3.7 nm
r
Median: 3.5 nm
p — ]. / •,
v v
, V‘‘4
B E )AuPd/C (Au Pd«7:l)
3 4 S 6 7P article Size (nm )
Mean: 3.9 nm Median 3.5 nm
8 9Particle Size (nm)
r ] AuPd/C (Au Pd«2 1)
1 2 3 4 5 6 7 8 9 10Particle Size (nm)
FTTvl AuPd/C (Au.M=l:l)
Mean: 4.9 nm Median: 4.6 nm
25
20
IS
10cr
S0
1 2 3 4 5 6 7 8 9 10 11Particle Size (nm)
C Z I AuPd/C (Au Pd-12)
Mean: 3-2 nm Median: 2.9 nm
2 3 4 5 6 7 8 9 10Particle Size (nm)
E*&»&ru. 5
r ~ji AuPd/C (Au:Pd»l:7)
Mean: 4.3 nm Median: 4.0 nm'
4 5 6 7 8 9Particle Size (nm)
10
Mean: 5.7 nm Median: 5.4 nm
3 4 5 6 7 8 9 10 11 12P article S ize (nm )
Figure 5.3 : Particle size distributions o f the AuPd particles (various mol ratios) immobilised on the activated carbon support. (Data supplied by Ram et at)
86
jp£erco3O’OJ
^ A u P d / C ( A u : P d . l:1 .8S|
P article Size (nm )
&hapUr S Q&obetttx^ree (Oxidation ofO&atqfd akohol
Table 5.1: M ean and m edian particle sizes o f the various immobilized Au-Pd sols
investigated in this study.
Au:Pd molarAu only 7:1 2:1 1:1 1:1.85 1:2 1:7
ratio
Mean size (nm ) 3.7 3.9 3.5 4.9 3.7 3.2 4.3
M edian size (nm) 3.5 3.5 3.2 4.6 3.3 2.9 4.0
Au:Pd molar ratio
2:1 1:1 1:2
Figure 5.4: Representative STEM-HAADF images showing AuPd hom ogeneous alloy particles. (a)(b), A u:Pd=2:l; (c)(d), A u :P d = l:l; (e)(f), A u:Pd= l:2 . (Im ages taken by Ram et al)
Figure 5.4 shows representative STEM -HAADF images o f the alloy particles in the
2:1, 1:1, and 1:2 Au-Pd samples. In each case, the metal particles were found to be
random hom ogeneous Au-Pd alloys with a face-centered cubic (fee) structure. If core
shell m orphologies, or any other gross form o f Au-Pd segregation, were present in these
nanoparticles, these would have been apparent by atomic mass (z) contrast in these
87
Pd
only
5.7
5.4
(Chapter S Q&obent free (Oxidation ofO&etizf cdcohol
HAADF images. It is interesting to note that the Au-Pd particles remained
pseudospherical (i.e., did not strongly wet the activated C support material).
-v;v
2 nm
(C)
V v :-
1 nm
' ^ '■:/ "*
2 nm
(b) ■“ 1
------
Auklap LaULa
—
r
4 1Ls
kt*V
(d) 500
tW
300
AuM i FdLa200\ ±
100
0
■r u
l»V
Figure 5.5: A set of HAADF images and corresponding XEDS point spectra from particles at the low (a,b), median (c,d) and high (e,f) ends of the particle size distribution for the Au:Pd (1:7) sample. They demonstrate a systematic particle size/composition variation, where the smaller particles progressively become more Au-rich and Pd- deficient. (Images taken by Ram et al)
Energy dispersive X-ray spectra were acquired from areas containing — 100 metal
particles in order to qualitatively check alloy compositions. The relative Au to Pd peak
areas from such “averaged” spectra agreed with the nominal x and y values for each of
the AuxPdy alloy compositions. Furthermore, XEDS spectra obtained from individual
particles showed characteristic Au M lines and Pd L lines confirming that intimately
88
Chapter S Qfbebentx^ree (Oxidation ofO&enzpl ako/iol
mixed AuPd alloys had been formed rather than physical mixtures o f pure Au and Pd
particles. However, it was noticeable that, within specific AuxPdy samples, the XEDS
spectra obtained from individual particles did show considerable systematic composition
variations as a function o f particle size.
For example, figure 5.5 shows a set o f HAADF images and corresponding XEDS
point spectra from particles at the low, median, and high ends of the particle size
distribution for the Au/Pd (1:7) sample. While all the particles were homogeneous AuPd
alloys, the smallest particles were Au-rich, whereas the largest ones tended to be Pd-rich.
It is interesting to note that this systematic composition/size trend is diametrically
opposite to the situation found for Au-Pd nanoparticles prepared by impregnation
methods where the larger particles tended to Au-rich and the smaller Pd-rich (23)
The Pd(3d) and Au(4f) X-ray photoelectron spectra for the series of catalysts are
shown in figures 5.6 and 5.7, respectively. A distinct shift in the Au(4f7/2) binding
energy is observed in the composition range Au/Pd=l:0 (binding energy=84.2 eV) to
A u/Pd=l:l (84.0 eV), being constant there after; this may reflect alloying with the Pd.
Analysis o f the Pd(3d) spectra is complicated by the severe overlap between the Pd(3d)
doublet and the Au(4ds/2) component. However, when we calculate the experimental
surface Pd/Au molar ratios based on the Pd(3d5/2)+Pd(3d3/2)+Au(4ds/2) combined
integrated intensities and the Au(4f) intensity together with known relative sensitivity
factors, we are able to correct the molar ratio by simply subtracting 0.5 from the raw
experimental values.
The corrected experimental ratios are plotted against nominal ratios (based on the
preparation recipes) in figure 5.8 and show a very clear linear correlation for Pd/Au
89
&haptor S Q&ob&tti free (Oxidation ofoScnzpl alcohol
ratios up to Pd/A u=l:l, and the X-ray photoelectron spectroscopy (XPS)-derived Pd/Au
molar ratio matches the ratios used in the synthesis. However, as we increase the relative
ratio o f Pd/Au in the synthesis above unity, XPS-derived surface ratios are consistently
approximately a factor o f 2 lower than “expected”. This could in principle arise from (i)
a systematic difference in alloy composition with particle size, (ii) the presence of
Pd(core)-Au(shell) nanoparticles, (iii) differential deposition of Au and Pd relative to the
intended composition, or (iv) preferential adsorption o f smaller Pd-rich nanoparticles
deeper into the carbon structure.
The TEM characaterization we have described supports explanation (i) as the most
likely explanation. The STEMHAADF/XEDS analysis rules out explanation (ii), and
TEM-XEDS analysis and visual inspection during the immobilization procedure rule out
any significant contribution o f (iii). An explanation based in (iv) would be in agreement
with similar suggestions in the literature (24) but still is purely speculative and lacks any
substantial evidence at this time.
Importantly, HAADF with XEDS point spectra analysis demonstrate that, for the
catalysts with a high Pd/Au ratio, a systematic particle size/composition variation
occurs. The larger particles progressively become more Pd-rich and Au-deficient, and
considering the high surface sensitivity o f XPS, this effect alone could provide a
satisfactory explanation for the systematic deviation observed between Pd/Au ratios used
in the synthesis and calculated from XPS data. The Pd nanoparticles comprise
predominantly Pd° species (Pd(3ds/2) binding energy ca. 335 eV) for Pd/Au ratios<l, but
the presence of Pd2+ species is indicated for higher Pd contents (figure 5.6), with the
highest concentration corresponding to the Pd-only catalyst.
90
(?liapur S Q&obent- freo (Oxidation ofO&onqpl alcohol
70 0 - Pd(3d),(a) Au(4d)+Pd(3d)650
Au.Pd600
550
500
•'I' 4502:1
400
350
300
7:1250
Au(4d)200
150 -
365 360 355 350 345 340 335 330 325
binding energy (eV)
1400. (b ) Au(4d)+Pd(3d)
Au:PdPd°1300 -
1200 -
0:1Pd1100 -
1000 -
900
80 0 -
700 -
1:1.856 0 0 -
365 360 345 340 325355 350 335 330
binding energy (eV)
Figure 5.6: Pd (3d) spectra for the series of Au-Pd/carbon catalysts; nominal Au:Pd ratios are as indicated.
9 /
(yhaptor S ^ obon t^ rce (£)xictation of O&onzpl alcohol
600 - (a) A u(4 f)
Au:Pd
0:1
1050
1000
900-
850-
800-
750 -
1:1.85650-
90 8692 88 84 82 6094
binding energy (eV)
Figure 5.7: Au(4f) spectra for the series of Au-Pd/carbon catalysts; nominal Au:Pd
ratios are as indicated.
92
Chapter S Q$bcbent-< free (Oxidation ofO&enzpl alcohol
O
23<-bQ_JOo E
~oa>>a>T3■coCLX
8
7
6
5
4
3
2
1
00 2 3 6 81 4 5 7
Nominal molar Pd:Au ratio
Figure 5.8: XPS-derived corrected Pd:Au molar ratios plotted as a function of nominal (expected) Au:Pd ratios. The dotted line shows the expected ratio.
The catalytic oxidation o f benzyl alcohol under free solvent conditions has been
chosen as an appropriate model reaction for exploring the catalytic performance of the
bimetallic Aux-Pdy catalysts. The results of the catalytic tests at 120 °C are presented in
Table 5.2.
93
(yltapta S Qtboberit freo (Oxidation ofoSon^d alcohol
Table 5.2: Effect o f the molar Au:Pd ratio on benzyl alcohol oxidation results with 1
wt% (Aux-Pdy)/C catalysts prepared by sol immobilisation method
Selctivity (%)
Catalyst Conv. (%) Toluene BenzaldehydeBenzoic
acid
Benzyl
benzoate
1% A u/C si 6.6 23.9 63.9 3.1 9.0
l%(7Au-lPd)/CSi 11.6 1.9 66.3 7.2 24.5
l%(3Au-lPd)/CSi 23.3 6.0 72.5 8.6 12.9
l% (2Au-lPd)/CSi 52.0 10.9 77.5 3.5 8.1
l% (lA u-lPd)/C Si 71.1 4.0 69.8 19.9 6.3
l%(lAu-1.85Pd)si 80.7 3.4 67 23.1 6.4
l% (lAu-2Pd)/CSi 90.8 4.5 67.4 22.9 5.1
l% (lAu-3Pd)/CSi 94.7 4.2 67 23.5 5.3
l% (lAu-7Pd)/CSi 88.7 1.7 68.4 24.1 5.8
l%Pd/CSi 59.3 6.8 74.7 10.4 8.1
Reaction conditions: T = 120 °C (393K), pOi = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml benzyl alcohol, catalyst - 0.05 g, t = 6 h.
The activity o f the monometallic 1 wt%Au/C was very poor, but on increasing the
Pd content we observed a progressive increase in catalytic activity. This increase of
activity reached a broad maximum with a Au/Pd molar ratio between 1:1 and 1:3, as
values above 95% conversion are achieved. A further increase in the Pd content resulted
in a progressive decrease in catalytic activity. It is important to note that even in the
presence of a minor amount o f gold or palladium (i.e., in the case of 1:7 and 7:1 Au/Pd
94
Chapter S QSxfhvnlx^ree (Oxidation ofoScnzpt alcohol
molar ratios) a significant increase in the catalytic activity was observed; specifically, an
increase in activity by a factor of 2.3-2.6 has been observed with respect to the
monometallic Au and Pd catalysts.
In order to determine the effect of the Au/Pd molar ratio on selectivity, the catalytic
data were compared at iso-conversion, and Table 5.3 shows the catalytic data at 50%
conversion.
Table 5.3: Comparison o f benzyl alcohol oxidation at iso-conversion o f 50% with 1
wt% (Aux-Pdy)/C catalysts prepared by colloidal method
Selctivity (%)
Catalyst Toluene BenzaldehydeBenzoic
acid
Benzyl
benzoate
l% (2Au-lPd)/CSi 11.0 78.6 3.5 6.9
l% (lA u-lPd)/C Si 5.4 79.7 7.4 7.5
l% (lAu-1.85Pd)CSi 4.3 81.9 6.6 7.2
l% (lAu-2Pd)/CSi 7.5 83.6 3.5 5.4
l% (lAu-3Pd)/CSi 6.6 85.4 2.4 5.5
l%(lAu-7Pd)/CSi 1.9 85.8 5.5 6.8
l%Pd/CSi 6.4 76.0 8.9 8.7
Reaction conditions: T = 120 °C (393K), pOj = 10 bar (0.1 MPa), stirring rate - 1500 rpm, substrate - 40 ml benzyl alcohol, catalyst - 0.05 g, t = 6 h. Catalysts containing higher amounts o f Au could not be compared at iso-conversion as the conversion with these catalysts was typically <50% for reasonable reaction times.
These data show that the catalytic performance in terms o f reaction specificity is not
particularly significant and all catalysts give high selectivities to benzaldehyde which is
Chapter S QS>ohvnt-( freo (Oxidation ofoSonzpt alcohol
the primary product (Scheme 5.1). Hence, the main effect of alloying Au with Pd is an
effect o f enhancing activity. Although at high conversions with Pd-rich catalysts the
selectivity to benzoic acid is enhanced. In particular, the formation of toluene, which is
generated from benzyl alcohol by a hydrogen transfer reaction, does not increase at iso
conversion as the Pd content increases. It has been shown from the H2O2 hydrogenation
studies that hydrogenation increases with Pd concentration in the Au-Pd alloy (25),
hence it is expected to observe higher toluene selectivities as the Pd content is increased.
To the contrary, selectivity values at iso-conversion indicate that the selectivity toward
toluene is highest for the gold-rich catalysts (Pd/Au=0.5).
CHO COOH
(D (2) (3 i2oh
Scheme 5.1: General reaction pathway for benzyl alcohol oxidation.
96
Chapter S Qfbohvrit- free (Oxidation ofoS em f dcohol
Figure 5.9 shows a plot o f turnover frequencies (TOF @ 0.5 h) obtained with the
increase o f Pd molar % in l%AuPd/C catalyst prepared by sol immobilisation. The TOF
value reaches maximum of >60 000 h*1 for the catalyst containing 65% Pd content out of
100% (Au+Pd), which is 1:1.85 Au/Pd catalyst, revealing it is the most active catalyst
for benzyl alcohol oxidation under the described reaction conditions.
60000-
50000-
40000 -
ULoI-
30000 -
20000 -
10000 -
200 40 60 80 100
P d m o la r%
Figure 5.9: TOF (at 0.5 h) trend for the increasing molar % o f Pd in l%AuPd/C, tested for benzyl alcohol oxidation
Using the particle size distribution obtained via TEM, it has been possible to
calculate the distribution o f atoms on the catalyst surface for the composition of Pd/Au =
1.85 which coincides with the optimized activity catalyst. Taking these values into
97
(Chapter S Qfbdtvntx^ree (Oxidation ofoScnzpl alcohol
account, the activities have been recalculated to use only those atoms exposed on the
catalyst surface. For benzyl alcohol oxidation, a TOF of 1 96 000 mol benzyl alcohol
reacted per surface atom of metal per hour is observed. This is further evidence o f the
very high activity that the Au-Pd alloy system can produce when the preparation method
and composition are properly optimized.
References:
(1) Bond, G. C.; Thompson, D. T. Catal. Rev.-Sci. Eng. 1999, 41, 319-388.
(2) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41-51.
(3) Haruta, M. Gold Bull. 2004, 37, 27-36.
(4) Hashmi, A. S. K. Gold Bull. 2004, 37, 51-65.
(5) Meyer, R.; Lemaire, C.; Shaikutdinov, Sh. K.; Freund, H.-J. Gold Bull. 2004, 37, 72-
124.
(6) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896- 7936.
(7) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405—408.
(8) Landon, P.; Ferguson, J.; Solsona, B. E.; Garcia, T.; Carley, A. F.; Herzing, A. A.;
Kiely, C. J.; Golunski, S. E.; Hutchings, G. J. Chem. Commun. 2005, 3385- 3387.
(9) Hutchings, G. J. J. Catal. 1985, 96, 292-295.
(10) Sinha, A. K.; Seelan, S.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43,
546-1548.
(11) Hughes, M. D.; Xu, Y.-J.; Jenkins, P.; McMom, P.; Landon, P.; Enache, D. I.;
Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin,
K.; Kiely, C. J. Nature 2005, 437, 1132-1135.
98
(Chapter 5 Qfcofoenti free (Oxidation ofO&onqd alcohol
(12) Abad, A.; Conception, P.; Corma, A.; Garcia, H. Angew. Chem., Int. Ed. 2005, 44,
4066-4069.
(13) Corma, A.; Serna, P. Science 2006, 313, 332-334.
(14) Edwards, J. K.; Solsona, B.; Ntainjua, E.; Carley, A. F.; Herzing, A. A.; Kiely, C.
J.; Hutchings, G. J. Science 2009, 323, 1037-1041.
(15) Edwards, J. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2008, 47, 9192-9198.
(16) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.;
Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science
2006,311,362-365.
(17) Bianchi, C. L.; Canton, P.; Dimitratos, N.; Porta, F.; Prati, L. Catal. Today 2005,
102-103, 203-212.
(18) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212^217.
(19) Lopez-Sanchez, J. A.; Dimitratos, N.; Miedziak, P.; Ntainjua, E.; Edwards, J. K.;
Morgan, D.; Carley, A. F.; Tiruvalam, R.; Kiely, C. J.; Hutchings, G. J. Phys. Chem.
Chem. Phys. 2008, 10, 1921-1930.
(20) Deki, S.; Akamatsu, K.; Hatakenaka, Y.; Mizuhata, M.; Kajinami, A. Nanostruct.
Mater. 1999, 11,59-65.
(21) Scott, R. W. J.; Wilson, O. M.; Oh, S.-K.; Kenik, E. A.; Crooks, R. M. J. Am.
Chem. Soc. 2004, 126, 15583-15591.
(22) Dash, P.; Bond, T.; Fowler, C.; Hou, W.; Coombs, N.; Scott, R. W. J. J. Phys.
Chem. C 2009, 113, 12719-12730.
(23) Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Edwards, J.; Conte, M.; Tang, Z. R.;
Hutchings, G. J. Faraday Discuss. 2008, 138, 337-351.
(24) Bianchi, C.; Porta, F.; Prati, L.; Rossi, M. Top. Catal. 2000, 13, 231-236.
(25) James Pritchard, Lokesh Kesavan, Marco Piccinini, Qian He, Ramchandra
Tiruvalam, Nikolaos Dimitratos, Jose A. Lopez-Sanchez, Albert F. Carley, Jennifer K.
99
Chapter S QfoobentT^free (Oxidation ofoSenspl alcohol
Edwards, Christopher J. Kiely, Graham J. Hutchings, Langmuir 2010, 26(21), 16568-
16577
100
Chapter 6ConcCusions and <Future
Chapter 6 Conclusions £< ( future G&brk
This chapter describes the conclusions derived from the experimental results,
emerged out o f this research project for the partial fulfilment of the completion o f the
PhD degree.
Selective oxidation o f primary carbon-hydrogen bonds with oxygen is of crucial
importance for the sustainable exploitation o f available feedstocks. To date,
heterogeneous catalysts have either shown low activity and/or selectivity or have
required activated oxygen donors. It is reported here that supported gold-palladium (Au
Pd) nanoparticles on carbon or TiC>2 are active for the oxidation o f the primary carbon-
hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl
benzoate under mild solvent-free conditions. Differences between the catalytic activity
o f the Au-Pd nanoparticles on carbon and TiC>2 supports are rationalized in terms of the
particle/support wetting behaviour and the availability o f exposed comer/edge sites.
Further these catalysts have been tested for substituted toluenes and benzyl alcohol, as
these substrates have similarity in their structural configuration. The results are
discussed with regard to structure activity relationship.
6.1. Conclusions:
> Blank toluene oxidation (without catalysts) has been done at 190 °C and 160 °C
using oxygen as an oxidant and found that there is a contribution o f homogeneous
reaction between toluene and oxygen at 190 °C, which is not observed at 160 °C.
Hence, it is concluded that reactions performed at 160 °C in the presence of catalysts
are purely heterogeneous in nature. This important observation suggests that the
earlier studies conducted at 190 °C (Table 6.1) may not in fact have been
heterogeneously catalyzed. The previous studies concerning toluene oxidation using
101
Chapter 6 Conclusions cSc (future Cftfiork
heterogeneous catalysts using O2 and tertiary butyl hydroperoxide (TBHP) is shown
in Table 6.1.
Table 6.1: Comparison o f toluene oxidation activity o f the Au-Pd supported catalysts with other reported catalytic systems.
Selectivity (%)
Catalyst T/P OxidantConv
(%)
Benzyl
alcohol
Benzal
dehyde
Benzoic
acid
Benzyl
benzoateTON
'Cu-Mn
(1/1)
190 °C
IMPa0 2 21.6 1.6 9.2 73.7 13.6 8
JCu-Fe/y-
AI2O3
190 °C
IMPa0 2 25.4 1.0 27.4 71.6 n.d. 74
3M nC 03190 °C
IMPa0 2 25.0 5.3 9.7 80.8 n.d. 50
4CoSBA-1580 °C
1 atmTBHP 8.0 n.d. 64.0 n.d. n.d. 103
5Cr/Silicalite80 °C
1 atmTBHP 18.4 5.2 23.3 25.7 n.d. n.d.
6AuPd/C
160 C
0.1
MPa
0 2 50.8 0.1 1.1 4.5 94.3 3300
n.d - not determined
> Support materials like carbon, titania have been tested for toluene oxidation at
160 °C and shown that they have no activity over toluene activation under given
reaction conditions. Therefore these supports have been chosen to immobilize
metal nanoparticles on top of them.
102
(Chapter 6 ffottchisions &C fu tu re Gtfbrk
> Titania supported Au-Pd nanoparticles prepared by the impregnation method has
been tested for toluene oxidation and found to have no remarkable activity since
the particle size was relatively large and particles had substantial compositional
variations.
> Carbon and titania supported Au-Pd alloy nanoparticles prepared by a sol
immobilization technique have been tested for toluene oxidation and observed to
have considerable conversion of toluene. Carbon based catalyst has shown twice
the activity o f the titania representative and it was been chosen for further
studies.
> A systematic set of carbon supported Au-Pd colloidal nanoparticles having a
range o f Au/Pd ratios (by keeping lwt% total metal) prepared by sol
immobilisation have been tested for toluene oxidation at 160 °C for 7 hrs and
established that Au/Pd mol ratio o f 1:2 is the optimum level of loading to get
high activation in toluene. For the sake of convenience, Au/Pd mol ratio of
1:1.85 (closc 10 2) or 1:1 wt ratio has been chosen to explore the toluene oxidation.
> Also, a physical mixture of the separate Au/C and Pd/C catalysts showed no
enhancement, highlighting the necessity of synergistic effect between Au and Pd.
> Attempts made to increase the conversion of toluene by i) increasing the mass of
the catalyst, ii) prolonging the reaction time, gave positive results and revealed
that there is no contribution of diffusion limitations.
> Toluene oxidation carried out using a lower substrate/metal ratio (~3250) for
longer hours ca. 11 Oh at 160 °C, promoted the conversion of toluene into 100%.
Only, the bimetallic catalyst yielded full conversion, whereas monometallic
ended up with maximum of 8%. Again, it is confirming the presence of
103
(Chapter 6 (Conclusions £< Qjfntnrc (W ork
synergistic effect between Au and Pd, which is vital in activating toluene
molecules. The selective product found at full conversion was benzyl benzoate
(-85% ).
> Toluene oxidation carried out using a very lower substrate/metal ratio (-1625) at
160 °C, reduced the reaction time drastically from 110 h to -30 h, by retaining
the complete conversion of toluene and the selectivity o f benzyl benzoate
> Toluene oxidation performed at 140 °C to increase the selectivity of benzyl
benzoate as the temperature could play a role o f tuning the selectivity distribution
o f the products. The experiment resulted with -95% yield o f benzyl benzoate as
per the expectation.
> Structure -activity relationship of Au-Pd bimetallic sol/toluene oxidation has
been detailed with the help of STEM HAADF technique.
> The bimetallic colloids have a mean particle size o f 2.9 nm and were found to be
homogeneous Au-Pd alloys.
> After deposition o f the colloids onto either activated amorphous carbon or TiC>2,
the particle size went to a modest increase say 3.9 and 3.7 respectively. It is due
to the low temperature drying process o f these materials.
> The difference in activity of carbon and titania supported catalysts was detailed
in terms o f particle wetting behaviour, coordination number sites and the
morphology o f the particles.
> Low temperature and low pressure toluene oxidation have been carried out and
they have shown poor activity.
> Finally it is concluded that carbon supported Au-Pd alloy nanoparticles prepared
by a sol immobilization technique can give significantly improved activity for
104
Chapter 6 Conclusions £< ( future C\X?ork
the oxidation of toluene under mild solvent-free conditions. These catalysts have
TONs that are a factor of ~30 greater than those of previous heterogeneous
catalysts for this reaction and also display a remarkably high selectivity to benzyl
benzoate.
> The rapid and high level formation o f benzyl benzoate could result through four
possible mechanistic pathways, which are
(i) coupling o f the benzaldehyde and the benzyl alcohol to give the hemiacetal,
followed by oxidation to the ester, benzyl benzoate.
(ii) direct thermal (catalyzed) dehydrative esterification between benzoic acid
and benzyl alcohol.
(iii) Cannizzaro reaction between benzyl alcohol and benzaldehyde via alkoxide
(iv) Tishchenko coupling of two benzaldehydes via “dimer”, directly to the
ester.
> Experiments have been designed to test for the importance of each pathway and
the results ruled out (ii), (iii), (iv), leaving (i) as a trusted route.
> Catalyst reusability studies have been done for toluene oxidation using carbon
and titania supported AuPd alloy nanoparticles prepared by sol immobilisation
and the results show that catalysts are reusable
> Investigation o f leaching of metals from the catalyst has been studied and the
results reveal that the catalyst is stable.
> To show the general applicability of the catalyst, substituted toluenes such as
methoxy toluenes, nitro toluenes, and xylenes were tested under the same
reaction conditions as that of toluene.
JOS
Chapter 6 Conclusions S t fu tu re Qtf?ork
> As matter o f relevancy with toluene, benzyl alcohol oxidation experiments have
been studied under solvent free conditions at 120 °C. The products found were,
toluene, benzaldehyde, benzoic acid and benzyl benzoate.
> A series o f carbon supported Au-Pd colloidal nanoparticles having a range of
Au/Pd ratios (by keeping lwt% total metal) prepared by sol immobilisation been
tested for benzyl alcohol oxidation at 120 °C for 6 hrs and established that Au/Pd
mol ratio of 1:2 is the optimum level o f loading to get high conversion of benzyl
alcohol as it was observed in toluene oxidation. Further, the characterizations of
the catalysts are well detailed.
106
Chapter 6 Conclusions £< ( future G&brk
6.2. Ongoing work:
There are few series o f experiments have been planned as a follow up study of
toluene and benzyl alcohol oxidation. Preliminary results have been obtained and
characterizations o f the catalysts have to be explored to understand the structure-activity
relationship o f the catalysts.
6.2.1. EfTect of amount of solvent medium in sol preparation:
In this study, a series of l%AuPd/C (1:1 mol) catalysts have been prepared using a
standard preparation protocol o f sol immobilisation, except the volume of water (solvent
medium) utilized. The standard amount of water for any sol immobilisation preparation
is 800 ml. Now, we have tried to prepare the catalysts by changing the amount of water
used, say 100 ml, 150 ml, 200 ml, 1500 ml, keeping all other preparation parameters
constant. The idea behind these preparations is to have the particle size distributions in
different windows, say 10-8 nm, 8-6 nm, 6-4 nm, 4- 2 nm, 2-0.5 nm and also different
shapes if possible, since the concentration o f the sol is different in each case.
These catalysts have been tested for benzyl alcohol oxidation and toluene oxidation.
The results are compared at identical reaction conditions (Table 6.2 & 6.3).
Table 6.2: Toluene oxidation at 160 °C
Selectivity %Conv _______________________________________
Variable Time h Benzyl Benz Benzoic Benzyl TOF TON%
alcohol aldehyde acid benzoate
100 ml1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
water
4.0 1.5 0.2 7.9 1.1 90.8 24.0 95.9
7.0 2.3 0.3 6.4 1.1 92.2 21.2 148.4
107
Chapter 6
150 ml
water
200 ml
water
800 ml
water
1500 ml
water
(y ’otwlnsions S t ( -future Q&brk
24.0 5.4 0.6 3.9 1.8 93.7 14.6 350.8
31.0 7.7 0.6 2.6 1.7 95.1 16.1 499.2
48.0 9.0 0.6 2.1 1.4 95.8 12.2 584.6
1.0 0.4 4.2 23.4 4.7 67.8 27.2 27.2
4.0 1.4 1.5 9.2 1.6 87.6 22.8 91.1
7.0 2.1 1.1 5.7 1.2 92.0 19.5 136.4
24.0 6.9 0.5 2.7 1.8 95.0 18.6 446.2
31.0 8.7 0.4 2.1 1.6 95.9 18.3 567.4
48.0 13.5 0.5 1.5 1.5 96.5 18.3 878.0
1.0 0.9 5.2 45.3 7.2 42.3 57.0 57.0
4.0 3.7 0.1 13.1 8.9 77.9 59.6 238.4
7.0 4.2 0.2 12.4 11.5 75.9 38.8 271.3
24.0 16.3 0.2 2.7 4.6 92.4 44.2 1059.8
31.0 18.5 0.3 2.5 2.9 94.3 38.8 1203.0
48.0 30.3 0.2 1.1 2.4 96.3 41.1 1970.8
1.0 0.7 1.5 65.6 14.5 18.4 43.1 43.1
4.0 4.2 0.2 14.6 17.7 67.5 67.7 271.0
7.0 6.5 0.1 10.2 16.1 73.6 60.0 419.8
24.0 18.7 0.3 5.8 14.1 79.8 39.1 1213.2
31.0 21.4 0.2 3.5 10.5 85.8 58.1 1393.6
48.0 32.8 0.3 2.4 11.8 85.6 44.4 2132.2
0.5 0.1 91.1 8.9 0.0 0.0 6.5 3.2
Chapter 6 Conclusions S t fu tu re QMPork
7.0 3.7 0.3 7.3 3.7 88.8 34.8 243.6
24.0 8.7 0.1 2.2 0.5 97.3 23.6 565.7
31.0 10.9 0.0 1.6 0.7 97.6 22.9 710.5
48.0 15.0 0.1 1.2 0.6 98.2 20.3 972.6
Reaction conditions: Toluene - 20 ml, / 0 2- 10 bar, T = 160 *C, t = 48 h, Catalyst - 0.4393 g of !%AuPd/C prepared from XXX ml o f water, substrate/metal mol ratio = 6500.
Table 6.3: Benzyl alcohol oxidation at 120 °C
Selectivity %Conv
Catalysts Time h% Toluene
Benz
aldehyde
Benzoic
acid
Benzyl
benzoate
TOF
100 ml water 0.5 13.5 14.3 84.7 0.1 0.9 31544.7
1.0 21.1 12.9 81.2 1.0 4.9 24719.6
2.0 32.3 11.5 83.1 1.7 3.6 18940.8
4.0 56.1 9.8 82.7 3.5 4.0 16443.1
6.0 78.5 8.1 80.3 7.9 3.8 15337.8
150 ml water 0.5 23.7 16.6 83.0 0.1 0.3 55563.2
1.0 41.2 14.4 85.2 0.1 0.2 48349.4
2.0 57.0 9.9 88.7 1.0 0.4 33432.2
4.0 83.7 7.7 89.0 2.8 0.4 24520.0
6.0 93.2 7.3 72.2 17.8 2.7 18217.6
200 ml water 0.5 17.9 15.6 81.2 0.4 2.8 41931.1
1.0 32.6 13.4 80.5 1.0 5.1 38236.4
2.0 54.5 11.1 82.1 2.1 4.7 31969.7
4.0 75.7 9.3 79.6 6.6 4.5 22196.7
6.0 90.6 8.0 65.7 22.1 4.3 17713.4
800 ml water 0.5 19.2 18.9 79.3 0.6 1.3 45016.1
109
Chapter 6 Conclusions &C fu tu re CM?ork
1500 ml water
1.0 26.8 15.5 82.2 0.9 1.4 31423.9
2.0 43.8 11.5 82.9 2.8 2.9 25690.0
4.0 68.6 8.5 78.0 7.9 5.6 20099.1
6.0 83.5 7.5 69.0 18.7 4.8 16315.6
0.5 12.6 11.1 85.3 1.1 2.5 29639.2
1.0 22.5 9.7 85.9 0.9 3.5 26354.2
2.0 34.8 6.4 91.1 1.2 1.3 20376.9
4.0 59.4 4.4 84.7 8.1 2.8 17422.2
6.0 85.9 4.0 75.9 18.0 2.2 16786.1
: Benzyl alcohol -- 40 ml, / 0 2- 10 bar, T = 120 °C, t = 6 h, Catalyst - 50 mgof 1 %AuPd/C prepared from XXX ml o f water
6.2.2. Effect of calcinations:
Treating the supported nanoparticles under various thermal conditions could result a
change in particle size distribution as the heat energy helps to mobilise the metal
particles on the surface o f the support. To evaluate this, we have tested the catalysts,
treated at different temperatures (under static air) say, 200 °C, 300 °C, and 400 °C for
toluene and benzyl alcohol oxidation (Table 6.4 & 6.5). The results are compared at
identical reaction conditions.
Table 6.4: Toluene oxidation at 160 °C
Selectivity %Time Conv
Catalystsh %
Benzyl
alcohol
Benz
aldehyde
Benzoic
acid
Benzyl
benzoate
TOF TON
l%AuPd/C 7.0 6.5 0.1 10.2 16.1 73.6 60.0 419.8
1:1 mol 24.0 18.7 0.3 5.8 14.1 79.8 39.1 1213.2
dried 31.0 21.4 0.2 3.5 10.5 85.8 58.1 1393.6
n o
Chapter 6 Conclusions S t fu tu re QtfPork
48.0 32.8 0.3 2.4 11.8 85.6 44.4 2132.2
l%AuPd/C 7.0 2.6 0.5 3.7 4.3 91.5 24.0 168.0
l:lm ol 24.0 7.6 0.4 1.5 1.0 97.1 20.7 496.8
200 °C calcin 31.0 9.1 0.5 1.6 1.0 97.0 19.1 591.9
48.0 13.6 0.4 1.4 7.9 90.2 18.5 886.7
l%AuPd/C 7.0 1.8 0.9 12.3 1.8 85.0 16.5 115.3
1:1 mol 24.0 4.3 0.7 3.1 0.8 95.5 11.8 283.3
300 °C calcin 31.0 5.4 0.5 2.5 0.6 96.4 11.3 351.2
48.0 8.6 0.4 1.7 0.9 97.0 11.6 557.7
l%AuPd/C 7.0 0.5 6.0 18.9 3.7 71.4 5.0 35.3
l:lm ol 24.0 3.0 5.9 9.3 2.9 81.9 8.0 192.8
400 °C calcin 31.0 3.5 4.1 5.1 2.3 88.5 7.3 227.2
48.0 5.4 2.8 4.1 1.5 91.5 7.4 353.7
l%AuPd/Ti02 7.0 1.9 1.3 3.3 4.9 90.4 16.3 114.4
1:1 mol 24.0 5.5 0.8 2.3 4.0 92.9 13.7 328.1
dried 31.0 6.8 1.0 2.1 0.6 96.3 13.1 404.6
48.0 10.5 0.3 2.2 2.2 95.4 12.9 618.9
1 %AuPd/Ti02 7.0 1.5 2.5 5.1 3.4 89.0 12.6 88.3
1:1 mol 24.0 5.2 1.9 2.3 2.3 93.6 12.8 306.4
200 °C calcin 31.0 6.2 2.3 2.3 1.1 94.3 12.0 371.0
48.0 11.0 1.4 1.5 0.9 96.2 13.5 647.9
1 %AuPd/TiC>2 7.0 2.5 2.5 4.0 3.8 89.6 21.3 149.4
1:1 mol 24.0 6.7 2.0 2.3 0.7 95.1 16.6 398.4
300 °C calcin 31.0 8.9 1.8 2.0 0.9 95.3 16.9 523.8
48.0 12.6 0.7 1.8 1.3 96.3 15.6 747.6
111
(Chapter 6 (Ponckisions &C fu tu re CWerk
l%AuPd/Ti02 7.0 1.2 15.8 28.5 17.9 37.9 13.6 6.8
l:lm ol 24.0 4.2 2.8 9.2 1.9 86.2 10.2 71.2
400 °C calcin 31.0 5.8 2.2 4.2 0.7 93.0 10.4 250.6
48.0 9.6 1.9 3.4 1.1 93.6 11.9 569.6
Reaction conditions: Toluene - 20 ml, /0 2- 10 bar, T = 160 *C, t = 48 h, Catalyst - 0.4393 g, substrate/metal mol ratio = 6500.
The activity trend for the toluene oxidation is as follows.
Carbon: dried > 200 C > 300 C > 400 C
T i02: dried ~ 200 C < 300 C > 400 C < dried
Table 6.5: Benzyl alcohol oxidation at 120 °C
CatalystsTime
h
Conv
%
Selectivity %
TOFToluene
Benz-
aldehyde
Benzoic
acid
Benzyl
benzoate
1 %AuPd/C 0.5 15.1 12.4 80.9 1.1 5.6 35431.0
l:lm ol 1.0 32.0 10.5 83.5 1.9 4.1 37514.0
dried 2.0 51.9 8.4 83.0 3.8 4.8 30453.5
4.0 76.0 6.6 77.4 11.8 4.2 22267.7
6.0 88.1 4.7 66.4 24.7 4.2 17220.6
1 %AuPd/C 0.5 20.9 7.7 85.0 2.2 5.2 49092.1
1:1 mol 1.0 47.6 6.2 85.8 3.0 5.0 55868.4
200 °C calcin 2.0 75.1 4.9 85.2 5.1 4.7 44002.7
4.0 87.5 4.9 72.5 17.6 4.9 25654.8
6.0 92.4 4.7 56.2 34.0 5.2 18053.0
1 %AuPd/C 0.5 2.4 0.8 91.1 0.9 7.2 5537.1
1:1 mol 1.0 3.9 1.3 88.9 2.4 7.4 4570.9
300 °C calcin 2.0 7.2 1.7 85.9 3.3 9.2 4199.9
112
(Chapter 6 (Conclusions £*C (future Cft?ork
4.0 22.9 5.9 82.8 4.3 6.9 6715.6
6.0 64.3 6.7 80.6 6.1 6.6 12570.6
1 %AuPd/C 0.5 1.0 2.7 82.4 7.0 7.8 2376.0
l:lm oi 1.0 1.6 5.6 86.3 2.3 5.8 1863.1
400 °C calcin 2.0 3.1 3.1 88.8 2.6 5.5 1820.6
4.0 8.3 3.8 87.0 2.9 6.3 2439.9
6.0 25.1 7.3 86.6 2.8 3.3 4895.9
1 %AuPd/TiC>2 0.5 11.5 10.5 88.8 0.6 0.0 26868.4
l:lm ol 1.0 20.2 9.2 90.6 0.2 0.0 23640.5
dried 2.0 31.0 7.4 92.0 0.5 0.1 18171.7
4.0 45.0 5.8 89.5 3.0 0.6 13185.0
6.0 64.8 6.2 89.0 4.0 0.8 12663.5
l%AuPd/TiC>2 0.5 10.7 8.1 91.7 0.1 0.1 25059.1
1:1 mol 1.0 19.4 7.5 92.2 0.2 0.1 22751.7
200 °C calcin 2.0 29.4 6.6 92.9 0.5 0.1 17223.7
4.0 48.0 5.9 92.1 1.5 0.5 14059.7
6.0 63.3 5.7 86.3 6.0 2.0 12361.1
l%AuPd/Ti02 0.5 14.9 6.8 92.9 0.3 0.1 34984.5
1:1 mol 1.0 20.7 5.5 93.8 0.4 0.3 24312.6
300 °C calcin 2.0 40.6 5.3 93.9 0.6 0.2 23829.6
4.0 54.8 4.7 89.4 3.4 2.5 16071.4
6.0 71.5 5.0 83.5 8.8 2.7 13981.5
l%AuPd/Ti02 0.5 5.8 3.7 95.6 0.2 0.5 13493.5
1:1 mol 1.0 8.4 3.2 95.4 0.5 0.8 9829.0
400 °C calcin 2.0 16.9 2.9 95.8 0.7 0.6 9923.4
4.0 38.3 3.3 95.6 0.8 0.3 11216.0
Chapter 6 (Pencktsiens S t fu tu re QMPork
6.0 58.3 3.2 87.1 4.8 4.9 11394.6
Reaction conditions: Benzyl alcohol - 40 ml, / 0 2-1 0 bar, T = 120 *C, t = 6 h, Catalyst - 50 mg
The activity trend for the benzyl alcohol oxidation is as follows.
Carbon: dried ~ 200 C > 300 C > 400 C
T i02: dried ~ 200 C < 300 C > 400 C
In TiC>2 supported catalysts, the agglomeration was very slow until 200 °C which
leads to an almost similar conversion level o f dried catalyst. At 300 °C, the capping
PVA molecules were getting decomposed. Hence, we will have naked particles which
correspond to high activity o f the catalysts, calcined at 300 °C. Further at 400 °C, the
particles start to agglomerate very fast since the stabiliser is absent. But this statement is
not true with carbon based catalysts. In carbon supported catalysts, the agglomeration
starts from 200 °C itself. The reason could be the better wetting surface behaviour of
TiC>2, which stops the mobilisation of the metal particles until 300 °C, whereas there is
no such wetting o f particles with carbon surface. The characterizations o f these catalysts
have to be explored to confirm the above conclusions.
6.23. Effect of support:
In this study, we have tried to immobilise AuPd bimetallic alloy nanoparticles on
various supports apart from conventional carbon and titania. These catalysts have been
evaluated for benzyl alcohol and toluene oxidation (Table 6.6 & 6.7). Again the
characterizations have to be done to understand the origin o f activity difference between
the catalsysts.
1 /4
Chapter 6 (Ponchisiatts £< qjfuture GXPork
Table 6.6: Benzyl alcohol oxidation at 120 °C
CatalystsTime
h
Conv
% Toluene
Selectivity %
Benz- Benzoic
aldehyde acid
Benzyl
benzoate
TOF
1 %AuPd/MgO 0.5 7.1 1.6 97.5 0.0 0.9 15067.1
(1:1 wt) 1.0 11.1 1.9 97 0.0 1.1 11818.9
2.0 15.2 2.8 95.9 0.1 1.2 8102.9
4.0 22.0 3.6 95 0.1 1.3 5873
6.0 30.5 3.9 94.9 0.4 0.9 5427.8
1 %AuPd/acid 0.5 31.3 12.9 84 0.4 2.6 66862.2
carbon 1.0 52.6 10.8 88.2 0.6 0.5 56219.4
(1:1 wt) 2.0 73.7 7.7 85.9 2.3 4.0 39391.1
4.0 90.6 6.7 84.6 4.5 4.2 24190.8
6.0 95.1 6.4 63.7 25.3 4.6 16938.5
0.25%Au0.25% 0.5 3.2 2.7 93.8 0.5 3.1 13805.4
Pd/MCM41 1.0 4.7 3.3 93.4 0.5 2.7 10092.3
2.0 8.2 4.6 91.5 1.0 2.9 8775.1
4.0 14.1 6.0 89.2 1.9 2.8 7543.4
6.0 21.2 6.2 87.7 2.9 3.2 7538.3
1 %AuPd/ 0.5 9.3 2.7 93.3 0.6 3.3 19841.8
hydroxyapatite 1.0 20.4 2.8 92.3 1.3 3.6 21806.9
(1:1 wt) 2.0 36.5 2.5 92.2 1.6 3.7 19469.7
4.0 55.2 1.9 91.9 2.3 3.8 14733.7
6.0 69.3 1.7 89.3 5.0 3.9 12338.5
l%AuPd/Si02 0.5 6.8 1.5 80.4 3.3 14.8 14488.4
(1:1 wt) 1.0 9.7 1.4 84.3 2.6 11.7 10382.6
IIS
Chapter 6 dPonchisions cSt (future Gtfbrk
2.0 15.7 1.5 81.4 4.6 12.5 8362.8
4.0 25.1 1.4 76.4 10.3 11.9 6716.7
6.0 33.0 1.5 77.5 10.3 10.6 5871.4
l%AuPd/Ce02 0.5 3.5 5.9 88.2 0.2 5.7 7464.5
(1:1 wt) 1.0 5.6 5.3 88.6 0.3 5.8 6016.7
2.0 7.7 4.8 88.2 0.5 6.4 4135.6
4.0 13.5 3.2 89.4 1.0 6.5 3608.8
6.0 17.8 3.1 89.3 1.2 6.4 3160.4
l%AuPd/Zr02 0.5 8.0 4.9 90.2 0.4 4.6 17147.6
(1:1 wt) 1.0 12.5 4.1 90 0.9 5.0 13388.1
2.0 17.0 3.6 89.7 1.6 5.1 9091.9
4.0 26.2 3.0 89.3 2.2 5.6 7009.6
6.0 33.9 2.7 88.6 3.2 5.4 6027.5
1 %AuPd/ 0.5 13.6 14.2 80.2 1.1 4.4 31871.8
Carbon 1.0 19.9 12.5 80 2 5.5 23293.9
nanotube 2.0 32.3 10.4 80.4 4 5.2 18952.4
(1:1 mol) 4.0 50.3 7.9 79.9 7.4 4.9 14745.4
6.0 63.4 6.8 77.9 10.7 4.6 12389.0
l%AuPd1 0.5 13.9 0.6 90.1 3.9 5.5 29603.1
graphite 1.0 19.5 0.9 90.7 3.2 5.2 20843.0
(1:1 wt) 2.0 29.3 1.0 89.6 4.2 5.2 15626.2
4.0 45.2 1.0 84.6 9.2 5.2 12081.4
6.0 55.4 0.8 80.3 13.8 5.1 9857.2
Reaction conditions: Benzyl alcohol - 40 ml, / 0 2- 10 bar, T = 120 °C, t = 6 h, Catalyst - 50 mg
116
Chapter 6 Conclusions cBt ( future Q tfork
Table 6.7: Toluene oxidation at 160 °C
CatalystsTime
h
Conv
%Benzyl
alcohol
Selectivity %
Benz- Benzoic
aldehyde acid
Benzyl
benzoate
TOF TON
1 %AuPd/acid 0.5 0.8 1.8 19.1 0.5 78.6 50.1 25
carbon 7.0 6.5 0.6 1.7 1.1 96.7 30.0 209.9
(1:1 wt) 24.0 16.3 0.3 0.9 0.4 98.5 22.1 529.8
31.0 21.1 0.3 0.8 1.3 97.6 22.1 684.6
48.0 30.5 0.2 0.6 1.1 98.0 20.6 989.7
72.0 47.7 0.2 0.5 0.7 98.5 21.5 1550.3
0.25%Au0.25% 1.0 0.9 1.3 2.9 0.0 95.9 60 60
Pd/MCM41 4.0 1.7 1.2 3.7 0.0 95.1 28 112
7.0 2.2 1.4 3.1 0.0 95.6 20.4 143.1
1 %AuPd/ 0.5 0.9 10.3 18.8 2.3 68.6 60.7 30.3
hydroxyapatite 7.0 4.4 2.7 3.4 0.7 93.2 20.4 142.6
(1:1 wt) 24.0 10.1 0.9 1.3 0.9 97.0 13.7 328.8
31.0 14.6 1.0 1.7 1.8 95.5 15.3 474.9
48.0 25.3 0.9 1.4 2.8 94.9 17.1 821.7
Reaction conditions: Toluene - 20 ml, TO2- 10 bar, T = 160 'C, t = 48 h, Catalyst - 0.4393 g,
substrate/metal mol ratio = 6500.
6.2.4. Oxidation of toluene using H2O2
There are few experiments designed to have alternate oxidants in place o f molecular
oxygen to oxidize toluene under high pressure. Obviously, the most desirable oxidant is
H2O2 since it is environmentally benign. Preliminary results (Table 6.8) have been
obtained and more attention is needed at this oxidation system as the product profile is
entirely different from the other system which uses molecular oxygen as an oxidant.
117
Chapter 6 (Ponchtsietis £< (^fiditre QtfPork
Table 6.8: Toluene oxidation using H2O2
ConvSelectivity %
Catalysts%
P- Benz- Hydro Phenyl p- TONUnknowns
C resol aldehyde quinone tolyl ether
5% AuPd/Ti02 83.5 6.2 26.3 5.0 18.1 44.3 325.7
(1:1 wt)a
ConvBenzene
Benz- HydroUnknowns TON
% aldehyde quinone
l%AuPd1C 57.6 19.87 13.3 32.4 34.4 169.1
(l:lw t)b
Reaction conditions: 10 ml o f toluene solution (0.02 g o f toluene in 50 ml water), Catalyst - a0.0276 g, b0.1381g, T = 50 °C, /?He = 0.3 Mpa, t = 0.5 h, Oxidant - 0.34 g H2O2, Stirring speed - 1500 rpm
118
Chapter 6 (Ponckisiotts &< q future Gtfbrk
References:
1. X. Li, J. Xu, L. Zhou, F. Wang, J. Gao, C. Chen, J. Ning, H. Ma, Catal. Lett. 110,
149 (2006)
2. F. Wang, J. Xu, X. Li, J. Gao, L. Zhou, R. Ohnishi, Adv. Synth. Catal. 347, 1987
(2005)
3. J. Gao, X. Tong, X. Li, H. Miao, J. Xu, J. Chem. Technol. Biotechnol. 82, 620
(2007)
4. R.L. Brutchey, I.J. Drake, A.T. Bell, T. Tilley, Chem. Commun. 3736 (2005)
5. A.P. Singh, T. Selvam, J. Mol. Catal. A: Chem. 113, 489 (1996)
119
£ iist o f Publications
LIST OF PUBLICATIONS
1. Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles
Lokesh Kesavan. Ramchandra Tiruvalam, Mohd Hasbi Ab Rahim, Mohd Izham bin Saiman,Dan I. Enache, Robert L. Jenkins, Nikolaos Dimitratos, Jose A. Lopez-Sanchez, Stuart H. Taylor, David W. Knight, Christopher J. Kiely, Graham J. Hutchings.Science, 331,2010, 195-199
2. Facile removal o f stabiliser-ligands from supported gold nanoparticlesJose A. Lopez-Sanchez, Nikolaos Dimitratos, Ceri Hammond, Gemma L. Brett, Lokesh Kesavan. Saul White, Peter Miedziak, Ramchandra Tiruvalam, Robert L. Jenkins, Albert F. Carley, David Knight, Christopher J. Kiely and Graham J. Hutchings.Nature Chemistry, 3, 2011, 551-556
3. Reactivity studies of Au-Pd supported nanoparticles for catalytic applications
Jose Antonio Lopez-Sanchez, Nikolaos Dimitratos, Neil Glanville, Lokesh Kesavan. Ceri Hammond, Jennifer K. Edwards, Albert F. Carley, Christopher J. Kiely and Graham J. Hutchings.Applied Catalysis A: General, 391, Issues 1-2, 2011, 400-406
4. Direct Synthesis of Hydrogen Peroxide and Benzyl Alcohol Oxidation Using Au-Pd Catalysts Prepared by Sol Immobilization
James Pritchard, Lokesh Kesavan. Marco Piccinini, Qian He, Ramchandra Tiruvalam, Nikolaos Dimitratos, Jose A. Lopez-Sanchez, Albert F. Carley, Jennifer K. Edwards, Christopher J. Kiely, and Graham J. Hutchings.Langmuir, 26 (21), 2010, 16568-16577